rustBuildCrate: properly handle cargo env pragmas with spaces

There are two problems: first that we end up splitting on spaces in the
loop. Even when that is fixed, we still would split on spaces in the
`export` inside the loop. We need to guard against both.

Fixes #199298

Confirmed that it fixes the case mentioned in the ticket:

```console
[nix-develop]$ $(nix-build -I nixpkgs=/home/shana/programming/nixpkgs Cargo.nix -A rootCrate.build  --no-out-link)/bin/nix-rustc-env-escape-repro
Expecting three words, got: first second third
```

I think this is going to cause a rebuild of every Rust package even if
they were unaffected, not much we can do here.

0 files changed, 0 insertions, 0 deletions