Write a `main.rs` to `src/bin/dummy` within the generated dummy source,
rather than `src/main.rs` to account for `illegalBin` test case
Signed-off-by: Roman Volosatovs <rvolosatovs@riseup.net>
* mkDummySrc: also include `no_std` attrs for target_os = uefi
* buildDepsOnly: only check with `--all-targets` if doCheck enabled
* mkDummySrc: allow customizing dummy Rust file contents
* Instead of injecting our own dummy `build.rs` file, we will patch the
`Cargo.toml` files to specify a build script in the Nix store
* This allow cargo to notice the difference (i.e. changed build script
path) where it could not before (due to nix enforcing that all sources
always have the same timestamp)
* There are some edge cases where cargo will allow a lib.rs file to be
present but not a main.rs (e.g. a binary cannot be called "examples",
which will happen if the crate is called that and a main.rs file is
present)
