prereq-build: add check for true and false

Some uses cases, like with autotools, need a path for 'true'
if we use them to force something to skip.

This will work by default on Linux hosts,
and require MacOS hosts to get coreutils,
which is currently installed in the CI
for 'macos-latest' host.

In the future, prereq stage can be reworked
to search for the actual binary
instead of relying on env.

Signed-off-by: Michael Pratt <mcpratt@pm.me>