Cargo-specific environment variables are missing

[tazjin]

Issue description

This issue was discovered in combination with the comrak issues
described in #2.

When building the comrak binary crate, the following line of code is
run (from here):

.about(crate_description!())

The crate_description! macro expands to a call to
env!("CARGO_PKG_DESCRIPTION"), which attempts to read the
CARGO_PKG_DESCRIPTION variable at compile-time.

However, this environment variable is not present in Carnix-builds
which will cause the build to error at that point.

In several of my projects I've applied a workaround for this that
uses overrideAttrs to "fake" the environment variable, as issue #2
meant that I could not figure out how to stop the binary from being
built:

comrak_0_2_9 = { features?(comrak_0_2_9_features {}) }: (comrak_0_2_9_ {
  # bla bla bla #
}).overrideAttrs (oldAttrs: rec { CARGO_PKG_DESCRIPTION = "dummy"; });

There may be other such environment variables that crates attempt to
use, but I haven't looked deeper into that yet.