CI: force MacOS jobs to use Intel runners (`macos-13`)

GitHub just switched macos-latest to the ARM chips (now alisasing
`macos-14`), and it brings a bunch of problems.

- First of all, GHC's 8.8 and 8.6 don't exist there.
- ghcup and llvm are unavailable

For the time being, lets stay on the previous version of the runner.

.github/workflows/bootstrap.yml

@@ -32,7 +32,7 @@ jobs:
         os: [ubuntu-latest]
         ghc: ["8.10.7", "9.0.2", "9.2.8", "9.4.8", "9.6.4", "9.8.2"]
         include:
-          - os: macos-latest
+          - os: macos-13
             ghc: "9.2.8"
     name: Bootstrap ${{ matrix.os }} ghc-${{ matrix.ghc }}
     runs-on: ${{ matrix.os }}

.github/workflows/validate.yml

@@ -66,7 +66,7 @@ jobs:
       GHC_FOR_RELEASE: ${{ format('["{0}"]', env.GHC_FOR_RELEASE) }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest, macos-13, windows-latest]
         # If you remove something from here.. then add it to the old-ghcs job.
         ghc: ['9.8.2', '9.6.4', '9.4.8', '9.2.8', '9.0.2', '8.10.7', '8.8.4', '8.6.5']
         exclude: