context: Add --env-fd option

This allows environment variables to be added to the context without
making their values visible to processes running under a different uid,
which might be significant if the variable's value is a token or some
other secret value.

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