ci: [refactor] Rename CIRRUS_PR env var to LINT_CI_IS_PR

The CIRRUS_PR env var was cirrus-specific and using a provider-agnostic
name makes more sense.

Also, enable pipefail, while touching this file.

This refactor is needed for the next commit.

.github/ci-lint-exec.py

@@ -35,13 +35,15 @@ def main():
         time.sleep(3)
         run(build_cmd)
 
-    CIRRUS_PR_FLAG = []
-    if os.environ.get("GITHUB_EVENT_NAME") == "pull_request":
-        CIRRUS_PR_FLAG = ["-e", "CIRRUS_PR=1"]
+    extra_env = []
+    if os.environ["GITHUB_EVENT_NAME"] == "pull_request":
+        extra_env = ["--env", "LINT_CI_IS_PR=1"]
 
     run([
-        "docker", "run", "--rm",
-        *CIRRUS_PR_FLAG,
+        "docker",
+        "run",
+        "--rm",
+        *extra_env,
         f"--volume={os.getcwd()}:/bitcoin",
         CONTAINER_NAME,
     ])

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 [ "$CIRRUS_REPO_FULL_NAME" = "bitcoin/bitcoin" ] && [ "${LINT_CI_IS_PR}" = "" ] ; 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