Make the Firefox source build reproducible, and let its check fail

The diff step rebuilds the extension from master-ff-src.zip (the source
archive AMO reviews) and compares the result with our build. It had never
worked, which the removed continue-on-error was hiding:

- The source zip left out .yarn/releases (yarnPath points there, so yarn
  could not start in the unpacked source) and scripts/ (build:ff runs
  scripts/postbuild-cleanup.cjs). Both are now included.
- diff.sh used `[ a || b ]`, which is not valid test syntax, and piped
  `diff -qr` into `sort`, so the check exited 0 whatever diff found. It now
  goes by diff's own exit status; with a file altered it fails.
- With those fixed, the builds still differed: webpack inlined
  import.meta.url from @pezkuwi/* packageInfo.js as the absolute file://
  path of the build machine, which also put our build directory into the
  published extension. DefinePlugin now gives it a fixed value; it is only
  shown in duplicate-version warnings.

Also fail-fast: false on the matrix. Without continue-on-error, the first
red step cancelled the others, so their results were lost.

Checked locally with node_modules as locked: "Builds are identical".

scripts/diff.sh

@@ -1,4 +1,4 @@
-# Copyright 2019-2025 @polkadot/extension-compat-metamask authors & contributors
+# Copyright 2019-2026 @pezkuwi/extension authors & contributors
 # SPDX-License-Identifier: Apache-2.0
 
 #!/bin/bash
@@ -9,14 +9,15 @@ yarn build:ff
 # Reorg the builds to 
 mkdir ff-diff
 
-OS=$(uname)
 FILE_PATH="./ff-diff"
 
 compare_directories() {
     local dir1=$1
     local dir2=$2
 
-    if diff -qr "$dir1" "$dir2" | sort; then
+    # diff's own exit status decides; piped into sort it was always 0, so builds
+    # that differed were reported as identical.
+    if diff -qr "$dir1" "$dir2"; then
         echo "Builds are identical"
         exit 0
     else
@@ -33,21 +34,8 @@ unzip_ff() {
     cd ./master-ff-src && yarn install && yarn build:ff && cd ..
 }
 
-if [ "$OS" == "Darwin" || "$RUNNER_OS" == "Linux" ]; then
-    echo "Running on macOS"
-    # macOS-specific commands go here
+mv ./master-ff-src.zip ./master-ff-build.zip "$FILE_PATH" && cd "$FILE_PATH"
 
-    mv ./master-ff-src.zip ./master-ff-build.zip $FILE_PATH && cd $FILE_PATH
+unzip_ff
 
-    unzip_ff
-    
-    compare_directories ./master-ff-build ./master-ff-src/packages/extension/build
-else # Assuming it will be Linux
-    echo "Running on Linux"
-    # Linux-specific commands go here
-
-    mv -t $FILE_PATH ./master-ff-src.zip ./master-ff-build.zip && cd $FILE_PATH
-    unzip_ff
-    
-    compare_directories ./master-ff-build ./master-ff-src/packages/extension/build
-fi
+compare_directories ./master-ff-build ./master-ff-src/packages/extension/build