Merge bitcoin/bitcoin#33888: ci: Re-enable LINT_CI_SANITY_CHECK_COMMIT_SIG

55555db055 doc: Add missing --platform=linux to docker build command (MarcoFalke)
fa0ce4c148 ci: Re-enable LINT_CI_SANITY_CHECK_COMMIT_SIG (MarcoFalke)
faa0973de2 ci: [refactor] Rename CIRRUS_PR env var to LINT_CI_IS_PR (MarcoFalke)
fa1dacaebe ci: Move lint exec snippet to stand-alone py file (MarcoFalke)

Pull request description:

  The sanity check to check the last few merge commit signatures on the main branch was accidentally and silently disabled while moving from the `cirrus-ci.com` platform to the GHA platform.

  So fix that by re-enabling it.

  Also, contains a few other lint cleanup commits.

ACKs for top commit:
  janb84:
    re ACK 55555db055
  willcl-ark:
    ACK 55555db055

Tree-SHA512: e623dc88035ee4d1c6a8efa5fad33c35cface87f54e78c7ebfe5d468d28d8d8097150344d276f90f8ed52a89e61609ce95380476ea0151b50f73ad5919233933

ci/lint/06_script.sh

@@ -6,19 +6,19 @@
 
 export LC_ALL=C
 
-set -ex
+set -o errexit -o pipefail -o xtrace
 
-if [ -n "$CIRRUS_PR" ]; then
+if [ -n "${LINT_CI_IS_PR}" ]; then
   export COMMIT_RANGE="HEAD~..HEAD"
   if [ "$(git rev-list -1 HEAD)" != "$(git rev-list -1 --merges HEAD)" ]; then
-    echo "Error: The top commit must be a merge commit, usually the remote 'pull/${PR_NUMBER}/merge' branch."
+    echo "Error: The top commit must be a merge commit, usually the remote 'pull/<PR_NUMBER>/merge' branch."
     false
   fi
 fi
 
 RUST_BACKTRACE=1 cargo run --manifest-path "./test/lint/test_runner/Cargo.toml"
 
-if [ "$CIRRUS_REPO_FULL_NAME" = "bitcoin/bitcoin" ] && [ "$CIRRUS_PR" = "" ] ; then
+if [ "${LINT_CI_SANITY_CHECK_COMMIT_SIG}" = "1" ] ; then
     # Sanity check only the last few commits to get notified of missing sigs,
     # missing keys, or expired keys. Usually there is only one new merge commit
     # per push on the master branch and a few commits on release branches, so