Makefile: Fix install target using doubly-prefixed Gitaly paths
With 17f3bf27 (Makefile: Use pattern rules to build Go binaries,
2022-05-10), we have converted the GITALY_EXECUTABLES
variable to
contain absolute paths instead of only the executables' basenames. The
install
target was still trying to prefix these binaries though, which
ultimately led to a path which had the binary directory's prefix twice
and thus broke installation.
Fix this bug by not reapplying the prefix.
Changelog: fixed