Filter out Bash read-only env vars in `export_env()` (such as `UID`)

[edmorley]

While looking at build failure metrics, I noticed this internal error:

-----> Installing dependencies using 'pip install -r requirements.txt'
/tmp/codon/tmp/buildpacks/0f40890b54a617ec2334fac0439a123c6a0c1136/vendor/buildpack-stdlib_v8.sh: line 56: UID: readonly variable
 !     Push rejected, failed to compile Python app.

This appears to be due to an app having the env var UID set in it's config vars, and UID being a read-only Bash env var.

We should filter this env var out (along with any others labelled as readonly on https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html), here: https://github.com/heroku/heroku-buildpack-python/blob/fc441449edad0a631352ad51cd10f54677a1c01b/vendor/buildpack-stdlib_v8.sh#L44-L60

We should probably do this before implementing #1451 / #1700.

GUS-W-19247022.