testhelper: Make gitaly-git2go symlink race-free
The compilation of binaries was made race-free in 16d52986 (testhelper: Refactor building of binaries for improved readability, 2021-10-08), but the creation of the symlink to the module-versioned path still suffered from races.
This change makes the symlink part of the compilation step, guarded with a sync.Once.
Closes: #3869 (closed)