CI: Update to macOS 12

Homebrew doesn't provide bottles for macOS 11 anymore, causing
dependencies to be compiled in CI which takes approximately 173 years.
Therefore, upgrade to macOS 12 and
- remove DEVELOPER_DIR environment variable which caused problems but
  its removal doesn't appear to break anything
- upgrade npm because versions before 10.2.2 use Python's distutils
  package which was removed in Python 12. See
    - https://github.com/npm/cli/pull/6937
    - https://github.com/nodejs/node-gyp/issues/2869

.github/workflows/build.yml

@@ -69,7 +69,7 @@ jobs:
           CCACHE_MAXSIZE: 500M
   macos:
     name: macos
-    runs-on: macos-11
+    runs-on: macos-12
     env:
       CMAKE_OPTS: >-
         -DUSE_WERROR=ON
@@ -78,7 +78,6 @@ jobs:
       CCACHE_MAXSIZE: 0
       CCACHE_NOCOMPRESS: 1
       MAKEFLAGS: -j3
-      DEVELOPER_DIR: /Applications/Xcode_11.7.app/Contents/Developer
     steps:
       - name: Check out
         uses: actions/checkout@v3
@@ -105,6 +104,7 @@ jobs:
       - name: Install dependencies
         run: |
           brew bundle install --verbose
+          npm update -g npm
           npm install --location=global appdmg
         env:
           HOMEBREW_NO_AUTO_UPDATE: 1