flutter: skip making Git repo deterministic

The build process for Flutter includes creating a Git repository, then
running the `make_deterministic_repo` function from Nixpkgs' `fetchgit`
on that repository.  However, the repository doesn't directly impact
anything in the final output, so there's no benefit to calling the
function.  Further, `make_deterministic_repo` doesn't work when using
versions of Git compiled with breaking changes (that is, the changes
that are currently opt-in, but will be required by Git v3).

Avoid the unnecessary work and work around `make_deterministic_repo`
being broken with some builds of Git by removing this step.

Fixes #575366

pkgs/development/compilers/flutter/flutter.nix

@@ -77,7 +77,6 @@ let
       GIT_AUTHOR_EMAIL= GIT_COMMITTER_EMAIL= \
       GIT_AUTHOR_DATE='1/1/1970 00:00:00 +0000' GIT_COMMITTER_DATE='1/1/1970 00:00:00 +0000' \
         git commit --allow-empty --message="Initial commit"
-      (. '${../../../build-support/fetchgit/deterministic-git}'; make_deterministic_repo .)
     ''
     + ''
       mkdir --parents bin/cache