setup-hooks/set-source-date-epoch-to-latest.sh: handle $path that starts with dash

Without the change `updateSourceDateEpoch` fails on tarballs like
`diffoscope-269` that contain single top-level `-269` root as:

    diffoscope> source root is -269
    diffoscope> find: unknown predicate `-269'
    diffoscope> chmod: invalid mode: ‘-269’
    diffoscope> Try 'chmod --help' for more information.

Currently `diffoscope-269` has a `sourceRoot = "./-269";` workaround to
bypass the failure.

Co-authored-by: Philip Taron <philip.taron@gmail.com>

pkgs/build-support/setup-hooks/set-source-date-epoch-to-latest.sh

@@ -1,5 +1,8 @@
 updateSourceDateEpoch() {
     local path="$1"
+    # Avoid passing option-looking directory to find. The example is diffoscope-269:
+    #   https://salsa.debian.org/reproducible-builds/diffoscope/-/issues/378
+    [[ $path == -* ]] && path="./$path"
 
     # Get the last modification time of all regular files, sort them,
     # and get the most recent. Maybe we should use