Prevent overrides of SHELLOPTS.

action.yml

@@ -42,6 +42,19 @@ outputs:
 runs:
   using: "composite"
   steps:
+    - id: env-setup
+      shell: bash
+      # Don't fail on error but respect pre-action env vars.
+      # We use the exit status as a conditional in lib.sh. 
+      # This is the default behavior but can be overriden by the caller in the SHELLOPTS env var.
+      run: |
+        OLDOPTS=$(set +o)
+        case $- in
+          *e*) OLDOPTS="$OLDOPTS; set -e";;
+          *) OLDOPTS="$OLDOPTS; set +e";;
+        esac
+        echo "CACHE_APT_PKGS_OLD_OPTS=$(cat $OLDOPTS)" >> $GITHUB_ENV
+        set +e
     - id: pre-cache
       run: |
         ${GITHUB_ACTION_PATH}/pre_cache_action.sh \
@@ -101,3 +114,8 @@ runs:
       run: |
         rm -rf ~/cache-apt-pkgs
       shell: bash
+
+    - id: env-teardown
+      shell: bash
+      run: |
+        eval $(cat $CACHE_APT_PKGS_OLD_OPTS)