offline flag disables `source-repository-package` sync (backport #9771)

How this was tested:

use the example project: https://github.com/peterbecich/cabal2nix-project-issue
- this example pulls the text library as a source-repository-package
build, install Cabal from this PR
- cabal install cabal --overwrite-policy=always
in the example project, delete dist-newstyle to remove the text library cached there
- rm -rf dist-newstyle; ~/.cabal/bin/cabal build
- text library pulled, build succeeds
next, build with the cached text library and offline flag
- ~/.cabal/bin/cabal build --offline
- this succeeds
- offline flag does not break the cached library
next, delete dist-newstyle again, and attempt build with --offline
- rm -rf dist-newstyle; ~/.cabal/bin/cabal build --offline
- this fails, as intended
- this proves the offline flag is preventing the text library from being pulled
- the error message could be improved:
```
$ rm -rf dist-newstyle; ~/.cabal/bin/cabal build --offline
.../dist-newstyle/src/text-8e814f5854032da8: getDirectoryContents:openDirStream:
does not exist (No such file or directory)
```

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This is an automatic backport of pull request #9771 done by Mergify.

haskell / cabal