Fix Makefile to avoid git describe error in CI

[ackintosh]

Issue Addressed

Fixes the error fatal: No names found, cannot describe anything. that occurs when running make
commands in CI (GitHub Actions).

https://github.com/sigp/lighthouse/actions/runs/19839541042/job/56844781126#step:5:13

fatal: No names found, cannot describe anything.

Proposed Changes

Changed the GIT_TAG variable assignment in the Makefile from immediate evaluation to lazy evaluation:

- GIT_TAG := $(shell git describe --tags --candidates 1)
+ GIT_TAG = $(shell git describe --tags --candidates 1)

This change ensures that git describe is only executed when GIT_TAG is actually used (in the build-release-tarballs target), rather than on every Makefile invocation.

Additional Info

Root cause:

• GitHub Actions performs shallow clones by default without fetching git tags
• The previous implementation using := (simple expansion) caused git describe to execute immediately when
  the Makefile was loaded, even for targets that don't use GIT_TAG (e.g., make install, make test)

[eserilev]

LGTM