Fix bashism in combine_distfiles_to_tar.sh.

This caused the following error message:

INFO: From Executing genrule //:bazel-distfile-tar:
./combine_distfiles_to_tar.sh: 44: [: Linux: unexpected operator
PiperOrigin-RevId: 344242407
diff --git a/combine_distfiles_to_tar.sh b/combine_distfiles_to_tar.sh
index 865a6d2..e54b26e 100755
--- a/combine_distfiles_to_tar.sh
+++ b/combine_distfiles_to_tar.sh
@@ -41,7 +41,7 @@
 done
 
 ID_OPTS="--group=0 --owner=0"
-if [ "$(uname -s)" == "Darwin" ]; then
+if [ "$(uname -s)" = "Darwin" ]; then
   ID_OPTS="--gid=0 --uid=0"
 fi