fix: hardcode main/master to always build (per review by @Mpdreamz)

Many repos keep legacy AsciiDoc docs on their default branch via the
do-not-delete_legacy-docs pattern in conf.yaml, so skip-build detection
is too risky to trust there. Bypassing legacy_branches.pl for main/master
also means the check no longer depends on the perl YAML module.

Author: reakaleek

.buildkite/scripts/build_pr.sh

@@ -41,16 +41,22 @@ buildkite-agent \
 
 
 if [[ "${GITHUB_PR_BASE_REPO}" != 'docs' ]]; then
-  # Skip the build but report success when the PR targets a branch that no longer carries
-  # legacy AsciiDoc docs (per conf.yaml). docs-build-pr is a required check, so we exit 0
-  # to keep it green rather than failing or leaving it pending.
-  # The helper exits non-zero on a parse error or unknown repo, which short-circuits the
-  # condition below so we build as today — i.e. fail open, never block a real build.
-  if legacy_branches=$(perl "$(dirname "$0")/legacy_branches.pl" "$GITHUB_PR_BASE_REPO") \
-       && [[ -n "${legacy_branches}" ]] \
-       && ! grep -qxF "${GITHUB_PR_TARGET_BRANCH}" <<< "${legacy_branches}"; then
-    echo "Target branch '${GITHUB_PR_TARGET_BRANCH}' has no legacy AsciiDoc docs in conf.yaml for ${GITHUB_PR_BASE_REPO} — skipping build (reporting success)."
-    exit 0
+  # main/master are hardcoded to always build, skipping the conf.yaml check below entirely:
+  # many repos keep legacy AsciiDoc docs on their default branch via the
+  # do-not-delete_legacy-docs pattern in conf.yaml, so this is the one case too risky to get
+  # wrong, and hardcoding it means it works even when the perl YAML dep is missing.
+  if [[ "${GITHUB_PR_TARGET_BRANCH}" != "main" && "${GITHUB_PR_TARGET_BRANCH}" != "master" ]]; then
+    # Skip the build but report success when the PR targets a branch that no longer carries
+    # legacy AsciiDoc docs (per conf.yaml). docs-build-pr is a required check, so we exit 0
+    # to keep it green rather than failing or leaving it pending.
+    # The helper exits non-zero on a parse error or unknown repo, which short-circuits the
+    # condition below so we build as today — i.e. fail open, never block a real build.
+    if legacy_branches=$(perl "$(dirname "$0")/legacy_branches.pl" "$GITHUB_PR_BASE_REPO") \
+         && [[ -n "${legacy_branches}" ]] \
+         && ! grep -qxF "${GITHUB_PR_TARGET_BRANCH}" <<< "${legacy_branches}"; then
+      echo "Target branch '${GITHUB_PR_TARGET_BRANCH}' has no legacy AsciiDoc docs in conf.yaml for ${GITHUB_PR_BASE_REPO} — skipping build (reporting success)."
+      exit 0
+    fi
   fi
 
   # Buildkite PR bot for repositories other than the `elastic/docs` repo are configured to