getdeps: correctly handle the install_dir value
getdeps: correctly handle the install_dir value

Summary:
We were computing `SCRATCH/install` and hashing based on that value,
but the build stuff was later computing `SCRATCH/installed` and passing that
to the builders.

Fixup the mismatch.

Reviewed By: simpkins

Differential Revision: D15337969

fbshipit-source-id: 70288f2d9286aaacf4c1f6e0dac4680a55edac6d

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro