rust: migrate to EAPI 7

This migrates the dev-lang/rust ebuild to use EAPI 7 instead of the
previous EAPI 5. EAPI 7 adds support for BDEPEND, which we intend
to use to get a build-time dependency on rust-bootstrap.

BUG=chromium:1122720
TEST=sudo emerge --unmerge rust && sudo emerge rust

Change-Id: I5f5b802e8df3a32a909028f45e5ce7acef8f6cc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2500585
Reviewed-by: Bob Haarman <inglorion@chromium.org>
Tested-by: Bob Haarman <inglorion@chromium.org>
Commit-Queue: Navil Perez <navil@google.com>
2 files changed