action: support openssl and sha256sum

As availability may be limited in some edge cases.

When a job is for example running within a container, `openssl` is not
always available. However, when running on a macOS or Windows runner,
the actual opposite is true.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

action/action.yml

@@ -81,7 +81,16 @@ runs:
           curl -fsSL -o "$DL_DIR/$FLUX_CHECKSUMS_FILE" "$FLUX_DOWNLOAD_URL/$FLUX_CHECKSUMS_FILE"
 
           echo "Verifying checksum"
-          sum=$(openssl sha1 -sha256 "$DL_DIR/$FLUX_TARGET_FILE" | awk '{print $2}')
+          sum=""
+          if command -v openssl > /dev/null; then
+            sum=$(openssl sha256 "$DL_DIR/$FLUX_TARGET_FILE" | awk '{print $2}')
+          elif command -v sha256sum > /dev/null; then
+            sum=$(sha256sum "$DL_DIR/$FLUX_TARGET_FILE" | awk '{print $1}')
+          else
+            echo "Neither openssl nor sha256sum found. Cannot calculate checksum."
+            exit 1
+          fi
+
           expected_sum=$(grep " $FLUX_TARGET_FILE\$" "$DL_DIR/$FLUX_CHECKSUMS_FILE" | awk '{print $1}')
           if [ "$sum" != "$expected_sum" ]; then
             echo "SHA sum of ${FLUX_TARGET_FILE} does not match. Aborting."