fix "cargo build --release"

[Wasabi375]

This bug only happend if you try to compile this library in release mode within the repo directory. This bug does not effect consumers of this library.

How to reproduce

1. Check out this repository
2. cargo build --release (it might be necessary to run cargo clean if the repo was compiled in release mode before)

What happens

After changing build.rs to include cmd.arg("-vv") in the build_uefi_bootloader function and also building using cargo build -r -vv --no-default-features --features uefi we can see that cargo hangs at the following step:

[bootloader 0.11.13]   Installing bootloader-x86_64-uefi v0.11.13 (/home/burkhard/programming/forks/bootloader/uefi)
[bootloader 0.11.13]     Updating crates.io index
[bootloader 0.11.13]     Blocking waiting for file lock on artifact directory

The issue is that the artifact directory for the uefi crate and for the bootloader crate are the same.

1. cargo build for the bootloader crate locks the artifact directory.
2. Then at the end of the build, build.rs is executed which tries to run cargo install bootloader-x86_64-uefi --path uefi ..... (the --path argument is only used when building localy and therefor this bug does not effect downstream consumers of this crate).
3. cargo install for the uefi subcrate tries to lock the same artifact directory

Impact

As far as I can tell it is impossible to run cargo test --release. Otherwise this is pretty harmless.

[Freax13]

LGTM, thanks!