depends: Make less assumptions about build env
Removes some implicit assumptions that the depends system has about its environment and, as a side-effect, makes it possible to build the depends tree under severely privilege-limited environments such as containers built by Guix.
Backport of core PR15581.
Test Plan: Run the Gitian builds.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5614