Fix Makefile to avoid git describe error in CI (#8513)

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.


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

```diff
- 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.


Co-Authored-By: ackintosh <sora.akatsuki@gmail.com>

Author: ackintosh

Makefile

@@ -3,7 +3,7 @@
 EF_TESTS = "testing/ef_tests"
 STATE_TRANSITION_VECTORS = "testing/state_transition_vectors"
 EXECUTION_ENGINE_INTEGRATION = "testing/execution_engine_integration"
-GIT_TAG := $(shell git describe --tags --candidates 1)
+GIT_TAG = $(shell git describe --tags --candidates 1)
 BIN_DIR = "bin"
 
 X86_64_TAG = "x86_64-unknown-linux-gnu"