Merge rust-bitcoin#2637: CI: Fix Manage PR job

35687c8 CI: Fix Manage PR job (Tobin C. Harding)

Pull request description:

  In rust-bitcoin#2635 we broke the Manage PR CI job but for some reason CI didn't run on that PR so we merged the breakage.

  Fix the script by setting default variables when they are not set. Done with ChatGPT.

ACKs for top commit:
  apoelstra:
    ACK 35687c8

Tree-SHA512: 707ad8872468a9eb6292ae3e8d7896a19baf044eaa28d6c45119d4367a7b14a73923a86cf68d64799df43febb35ecccb39422ce0d28f37100c6120a4316240d1

contrib/gen_label_config.sh

@@ -7,14 +7,10 @@ set -euo pipefail
 
 config=.github/labeler.yml
 
-if [ -n "$SCAN_DIR" ];
-then
-	scan_dir="$SCAN_DIR"
-else
-	scan_dir=.
-fi
+# Define a default value for SCAN_DIR if not set
+: "${SCAN_DIR:=.}"
 
-if [ "$1" '!=' "--force" ] && ! git diff --exit-code "$config";
+if [ "${1:-default}" '!=' "--force" ] && ! git diff --exit-code "$config";
 then
 	echo "Error: $config is not committed."
 	echo "Refusing to overwrite it to prevent disaster."
@@ -24,7 +20,7 @@ fi
 
 excluded_crates="fuzz|dep_test"
 
-CRATES="`cd "$scan_dir" && cargo metadata --no-deps --format-version 1 | jq -j -r '.packages | map(.manifest_path | rtrimstr("/Cargo.toml") | ltrimstr("'$PWD'/")) | join(" ")'`"
+CRATES="`cd "$SCAN_DIR" && cargo metadata --no-deps --format-version 1 | jq -j -r '.packages | map(.manifest_path | rtrimstr("/Cargo.toml") | ltrimstr("'$PWD'/")) | join(" ")'`"
 
 for crate in $CRATES;
 do