Get rid of make warnings in bb-install (#31587)
There isn't much reason to attempt a staged install, since we're essentially
just doing the second half of the staging (the first half being done on the BB builder).
Fixes annoying make warnings like:
```
/home/keno/julia/deps/libgit2.mk:114: warning: overriding recipe for target '/home/keno/julia/usr-staging/LibGit2.v0.27.7.tgz'
/home/keno/julia/deps/libgit2.mk:114: warning: ignoring old recipe for target '/home/keno/julia/usr-staging/LibGit2.v0.27.7.tgz'
```
