portal: Do not use caller-supplied variables in environment

If the caller specifies a variable that can be used to inject arbitrary
code into processes, we must not allow it to enter the environment
block used to run `flatpak run`, which runs unsandboxed.

This change requires the previous commit "context: Add --env-fd option",
which adds infrastructure used here.

To be secure, this change also requires the previous commit
"run: Convert all environment variables into bwrap arguments", which
protects a non-setuid bwrap(1) from the same attack.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Part-of: https://github.com/flatpak/flatpak/security/advisories/GHSA-4ppf-fxf6-vxg2