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.

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