cirrus: Introduce SKIP_TASK_ON_PR

Limit the number of tasks we run on PRs to only a subset instead of
the full set to reduce the amount of resources we use for PRs. Labeling
a PR with "fullci" can be used enable running all tasks.

This removes a total of 14 task execution from PRs by default at the cost of
needing to monitor master builds more closely for distro specific breakage.

This could probably be even be more advanced:

Leveraging changesIncludeOnly('scripts/*', 'testing/*'), we might be able to
limit to running a single Linux distro rather than a whole set of different
for simple script changes. For now I've left that out, however.

.cirrus.yml

@@ -66,6 +66,12 @@ branch_whitelist: &BRANCH_WHITELIST
       )
     ) )
 
+skip_task_on_pr: &SKIP_TASK_ON_PR
+  # Skip this task on PRs if it does not have the fullci label,
+  # it continues to run for direct pushes to master/release.
+  skip: >
+    ($CIRRUS_PR != '' && $CIRRUS_PR_LABELS !=~ '.*fullci.*')
+
 ci_template: &CI_TEMPLATE
   << : *BRANCH_WHITELIST
 
@@ -136,13 +142,6 @@ env:
 # Linux EOL timelines: https://linuxlifecycle.com/
 # Fedora (~13 months): https://fedoraproject.org/wiki/Fedora_Release_Life_Cycle
 
-fedora36_task:
-  container:
-    # Fedora 36 EOL: Around May 2023
-    dockerfile: ci/fedora-36/Dockerfile
-    << : *RESOURCES_TEMPLATE
-  << : *CI_TEMPLATE
-
 fedora37_task:
   container:
     # Fedora 37 EOL: Around Dec 2024
@@ -150,6 +149,14 @@ fedora37_task:
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
 
+fedora36_task:
+  container:
+    # Fedora 36 EOL: Around May 2023
+    dockerfile: ci/fedora-36/Dockerfile
+    << : *RESOURCES_TEMPLATE
+  << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
+
 centosstream9_task:
   container:
     # Stream 9 EOL: Around Dec 2027
@@ -163,6 +170,7 @@ centosstream8_task:
     dockerfile: ci/centos-stream-8/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
 
 centos7_task:
   container:
@@ -170,6 +178,7 @@ centos7_task:
     dockerfile: ci/centos-7/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
 
 debian11_task:
   container:
@@ -185,6 +194,7 @@ debian11_static_task:
     dockerfile: ci/debian-11/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
   env:
     ZEEK_CI_CONFIGURE_FLAGS: *STATIC_CONFIG
 
@@ -194,6 +204,7 @@ debian10_task:
     dockerfile: ci/debian-10/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
 
 opensuse_leap_15_4_task:
   container:
@@ -208,6 +219,7 @@ opensuse_tumbleweed_task:
     dockerfile: ci/opensuse-tumbleweed/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
 
 ubuntu2210_task:
   container:
@@ -215,6 +227,7 @@ ubuntu2210_task:
     dockerfile: ci/ubuntu-22.10/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
 
 ubuntu22_task:
   container:
@@ -234,6 +247,7 @@ ubuntu20_task:
     dockerfile: ci/ubuntu-20.04/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
 
 ubuntu18_task:
   container:
@@ -241,6 +255,7 @@ ubuntu18_task:
     dockerfile: ci/ubuntu-18.04/Dockerfile
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
 
 alpine_task:
   container:
@@ -266,6 +281,7 @@ macos_monterey_task:
   prepare_script: ./ci/macos/prepare.sh
   << : *CI_TEMPLATE
   << : *MACOS_ENVIRONMENT
+  << : *SKIP_TASK_ON_PR
 
 # FreeBSD EOL timelines: https://www.freebsd.org/security/#sup
 freebsd14_task:
@@ -288,6 +304,7 @@ freebsd13_task:
 
   prepare_script: ./ci/freebsd/prepare.sh
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
   << : *FREEBSD_ENVIRONMENT
 
 freebsd12_task:
@@ -298,6 +315,7 @@ freebsd12_task:
 
   prepare_script: ./ci/freebsd/prepare.sh
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
   << : *FREEBSD_ENVIRONMENT
 
 asan_sanitizer_task:
@@ -322,6 +340,7 @@ ubsan_sanitizer_task:
     << : *SANITIZERS_RESOURCE_TEMPLATE
 
   << : *CI_TEMPLATE
+  << : *SKIP_TASK_ON_PR
   test_fuzzers_script: ./ci/test-fuzzers.sh
   env:
     CXXFLAGS: -DZEEK_DICT_DEBUG
@@ -363,3 +382,4 @@ windows_task:
     # Give verbose error output on a test failure.
     CTEST_OUTPUT_ON_FAILURE: 1
   << : *BRANCH_WHITELIST
+  << : *SKIP_TASK_ON_PR