executors: Inject Certs and Env Vars

[Piszmog]

Today, it is quite difficult to inject certificates and global env vars into the containers spawned by Executors. There are workarounds to deal with this (creating custom images on top of Sourcegrah's images), but these are not ideal and create more maintenance for users.

We need to investigate how to inject consistent certs and env vars.

Done

• Determine the "best" way to set certs
  • There are numerous places certs can go (based on the OS), how do we determine where to put certs?
• Determine how to set env vars
  • Some script that runs prior to actual work that sets env vars?

[varungandhi-src]

Related:

• https://github.com/sourcegraph/sourcegraph/issues/49930
• https://github.com/sourcegraph/sourcegraph/pull/49732
• https://github.com/sourcegraph/sourcegraph/pull/56434