install-from-bindep needs PIP_OPTS

The install-from-bindep script should also incorporate `PIP_OPTS="${PIP_OPTS-}"` as is already done in the assemble script to allow for custom pip options.

Current workaround is to set your own `ARG CONSTRAINTS=` in a Containerfile to use that var, which could get overwritten by the builder

Start with the install-from-bindep script and compare how the assemble script handles PIP_OPTS. Confirm the existing bindep installation path and the current Containerfile workaround, then verify that custom pip options are preserved without relying on an overwritten ARG.

Written by the indexing model from the issue text.