CI: Fix Manage PR job

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.

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