GHA/macos: bump runner to macos-26 in clang-tidy jobs

Tiny difference in practice:
Apple clang 17.0.0.17000013 -> 17.0.0.17000603

To use the most recent tools for static analyses.

Closes #20745

.github/workflows/macos.yml

@@ -201,7 +201,7 @@ jobs:
 
   macos:
     name: "${{ matrix.build.generate && 'CM' || 'AM' }} ${{ matrix.build.compiler }} ${{ matrix.build.name }}"
-    runs-on: macos-15
+    runs-on: ${{ matrix.build.image || 'macos-15' }}
     timeout-minutes: 15
     env:
       DEVELOPER_DIR: "/Applications/Xcode${{ matrix.build.xcode && format('_{0}', matrix.build.xcode) || '' }}.app/Contents/Developer"
@@ -270,6 +270,7 @@ jobs:
             install: libnghttp3 libngtcp2 gsasl rtmpdump
             generate: -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DCURL_USE_GSASL=ON -DUSE_LIBRTMP=ON -DUSE_APPLE_IDN=ON -DUSE_NGTCP2=ON -DCURL_DISABLE_VERBOSE_STRINGS=ON -DUSE_APPLE_SECTRUST=ON
           - name: 'MultiSSL AppleIDN clang-tidy +examples'
+            image: macos-26
             compiler: clang
             install: llvm gnutls nettle libressl krb5 mbedtls gsasl rustls-ffi rtmpdump libssh fish
             install_steps: skiprun
@@ -284,6 +285,7 @@ jobs:
               -DCURL_COMPLETION_FISH=ON -DCURL_COMPLETION_ZSH=ON
 
           - name: 'HTTP/3 clang-tidy'
+            image: macos-26
             compiler: clang
             install: llvm libnghttp3 libngtcp2 openldap krb5
             install_steps: skipall