dmd,dtools,dub: 2.079.0 -> 2.079.1 and wrap ldc2 binary with $CC

(cherry picked from commit 4aa04d1)

Reason: This bumps the version to a newer release and fixes package
issues.

This project does not have a default versioning scheme. go2nix
suggests using the date of the most recent change.

(cherry picked from commit ab50043)

The Datadog agent requires `gohai` to be available on its `$PATH` in
order to collect certain metrics.

It would previously start up and collect certain types of metrics, but
log errors related to the missing gohai binary.

This commit configures the systemd-unit to make gohai available at
runtime.

This fixes #39810.

(cherry picked from commit f4c8718)

Darwin build succeeded locally but failed on Hydra,
see discussion in #41008.

This reverts commit 849e261.

(cherry picked from commit db2da80)