Trybuild fails if `Cargo.lock` is read-only

[MingweiSamuel]

In the (admittedly unusual) situation where the Cargo.lock in a project is read-only, trybuild copies the file including the read-only permissions and later errors due to this:

Caused by:
  failed to open: ..../cargo-target/tests/trybuild/my-package/Cargo.lock

Caused by:
  Permission denied (os error 13)

Will make a PR to fix