common-updater-scripts: Use custom fake temporary hash

Some update scripts (e.g. sublime4) already use all-zeroes hashes
for resetting a version when updating multiple sources.
When `update-source-version "${attrPath}" 0 "${lib.fakeSha256}"`
was executed, u-s-v would not be able to detect that the hash
changed and would fail with the following message:

    Failed to replace temporary source hash of '…' to the final source hash!

The update script worked when we tested it because change detection itself
was broken until recently: https://github.com/NixOS/nixpkgs/pull/190554

pkgs/common-updater/scripts/update-source-version

@@ -177,15 +177,15 @@ elif [[ "$oldHashAlgo" = "null" ]]; then
 fi
 
 case "$oldHashAlgo" in
-    # Lengths of hex-encoded hashes
-    sha256) hashLength=64 ;;
-    sha512) hashLength=128 ;;
+    # Choose a temporary hash for given algorithm.
+    # Not using all-zeroes hash, since that is sometimes
+    # used for clean-up when updating multi-source packages.
+    # Created by hashing “update-source-version” string.
+    sha256) tempHash=AzH1rZFqEH8sovZZfJykvsEmCedEZWigQFHWHl6/PdE= ;;
+    sha512) tempHash=KFj9Fvco4AuCgLJIGRnVzyssRf7VGP2oi5CkH6ADvj75ow3am3h8pxefOgQlO+i33Q/BBnG/ST/F7B/0BvWHxw== ;;
     *) die "Unhandled hash algorithm '$oldHashAlgo' in '$attr'!" ;;
 esac
 
-# Make a temporary all-zeroes hash of $hashLength characters
-tempHash=$(printf '%0*d' "$hashLength" 0)
-
 if [[ -n "$sri" ]]; then
     # SRI hashes only support base64
     # SRI hashes need to declare the hash type as part of the hash