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:

heroku-buildpack-python/vendor/buildpack-stdlib_v8.sh

Lines 44 to 60 in fc44144

	# Usage: $ export-env ENV_DIR WHITELIST BLACKLIST
	# Exports the environment variables defined in the given directory.
	export_env() {
	local env_dir=${1:-$ENV_DIR}
	local whitelist=${2:-''}
	local blacklist
	blacklist="$(_env_blacklist "$3")"
	if [[ -d "$env_dir" ]]; then
	# Environment variable names won't contain characters affected by:
	# shellcheck disable=SC2045
	for e in $(ls "$env_dir"); do
	echo "$e" | grep -E "$whitelist" | grep -qvE "$blacklist" \
	&& export "$e=$(cat "$env_dir/$e")"
	:
	done
	fi
	}

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

[edmorley]

We also need to filter out the env var RANDOM, since it's a special Bash env var too, otherwise RANDOM=Foo results in eg:

/tmp/codon/tmp/buildpacks/0f40890b54a617ec2334fac0439a123c6a0c1136/vendor/buildpack-stdlib_v8.sh: line 56: Foo: unbound variable

[edmorley]

Also non-ASCII env var names apparently (plus it seems API should prevent setting these in the first place?):

/tmp/codon/tmp/buildpacks/0f40890b54a617ec2334fac0439a123c6a0c1136/vendor/buildpack-stdlib_v8.sh: line 56: export: `<some-non-ascii-env-var>': not a valid identifier