Merge bitcoin/bitcoin#26588: ci: Skip COMMIT_RANGE if no CIRRUS_PR

fad1c55301 lint: Skip COMMIT_RANGE if no CIRRUS_PR (MarcoFalke)

Pull request description:

  It doesn't make sense to run this for non-PRs, because:

  * There are known whitespace "violations" in previous commits, so the lint may fail
  * Once the changes are merged, it is too late to fix them up (force pushes are illegal)
  * It isn't possible to determine which commits to run on if there is no reference branch (target branch of the pull request)

  Moreover, the test fails on non-master:

  * https://github.com/bitcoin/bitcoin/runs/8664441400

  Fix all issues by skipping it.

ACKs for top commit:
  hebasto:
    ACK fad1c55301, also tested in my personal Cirrus account.

Tree-SHA512: be15f00e2b2a9069583833545883e0e5968a33d2455dad59e6fb47c1102b4dd16ef932e9ba945e29e9d941e6c17bd531a02c66b0491097801be6bda476875537

ci/lint/06_script.sh

@@ -9,7 +9,12 @@ export LC_ALL=C
 GIT_HEAD=$(git rev-parse HEAD)
 if [ -n "$CIRRUS_PR" ]; then
   COMMIT_RANGE="${CIRRUS_BASE_SHA}..$GIT_HEAD"
+  echo
+  git log --no-merges --oneline "$COMMIT_RANGE"
+  echo
   test/lint/commit-script-check.sh "$COMMIT_RANGE"
+else
+  COMMIT_RANGE="SKIP_EMPTY_NOT_A_PR"
 fi
 export COMMIT_RANGE
 
@@ -36,8 +41,3 @@ if [ "$CIRRUS_REPO_FULL_NAME" = "bitcoin/bitcoin" ] && [ "$CIRRUS_PR" = "" ] ; t
     ${CI_RETRY_EXE} gpg --keyserver hkps://keys.openpgp.org --recv-keys "${KEYS[@]}" &&
     ./contrib/verify-commits/verify-commits.py;
 fi
-
-if [ -n "$COMMIT_RANGE" ]; then
-  echo
-  git log --no-merges --oneline "$COMMIT_RANGE"
-fi