This reverts commit deb7815.

Mixing up two distinct phases of a derivation's build is not a good idea. See
also #91689 (comment).

While the artifacts from `buildPhase` should be used for testing as
well, it should be avoided that those are modified during `checkPhase`.

This can happen if a package is built e.g. with special
`cargoBuildFlags` that don't apply to the `checkPhase`. In that case, a
binary would be installed into `$out` without those flags since
`checkPhase` overrides the binary in the `target`-directory.

This patch copies the state of `target/release` into a temporary
location at the end of the `buildPhase` and installs the results from
that temporary directory into `$out` while `checkPhase` can continue
using the configured build-dir.

cc #91689
Closes #93119
Closes #91191

rust: Fix build flags