Bump OS X Stack LTS from 12.26 to 18.28

This has been failing since 2024-05-03:
https://github.com/haskell/directory/actions/runs/9152501865/job/25160038949

    Error: [S-9443]
           No setup information found for ghc-8.4.4 on your platform. This probably means a GHC binary
           distribution has not yet been added for OS key macosx-aarch64. Supported versions:
           ghc-8.10.5, ghc-8.10.6, ghc-8.10.7, ghc-9.0.2, ghc-9.2.1, ghc-9.2.2, ghc-9.2.3, ghc-9.2.4,
           ...

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { os: macos-latest, stack: lts-12.26, stack-extra-deps: "bytestring-0.11.3.0, filepath-1.4.100.0, unix-2.8.0.0" }
+          - { os: macos-latest, stack: lts-18.28, stack-extra-deps: "bytestring-0.11.3.0, filepath-1.4.100.0, unix-2.8.0.0" }
           - { os: macos-latest, stack: lts-22.7, stack-extra-deps: "bytestring-0.11.5.3, filepath-1.5.2.0, os-string-2.0.2, unix-2.8.5.1", stack-package-flags: "{directory: {os-string: true}, unix: {os-string: true}}", ghc-flags: -Werror=deprecations }
           - { os: ubuntu-latest, ghc: 8.4.4, cabal: 3.0.0.0, overrides: "before_prepare() { sed -i.bak /utimensat/d configure.ac; }" }
           - { os: ubuntu-latest, ghc: 8.6.5, cabal: 3.0.0.0, overrides: "before_prepare() { sed -i.bak /utimensat/d configure.ac; }" }