TBR: lib/cmdline/testdata/gendoc: improve gendoc performance.
TBR: lib/cmdline/testdata/gendoc: improve gendoc performance.

- Use "go install" instead of "go build" to build binaries. This allows us to
  reuse intermediate objects.
- Use the default binary name (base name of its package) instead of gendoc.
  Since we build each binary in a different tmp location with different names,
  we can remove the "-a" flag which will also improve performance (not building
  everything from scratch).

The presubmit test of this CL shows that "vanadium-go-generate" took 7 minutes
to finish, which is a big improvement compared to the current 26 minutes run
time.

PresubmitTest: none
Change-Id: Ia09197b13a3d9a74ad6d49391c8cbf836b4d1d66

vanadium/go.lib