Re-declare GIT_VERSION inside the build stage so it expands

Re-declare GIT_VERSION inside the build stage so it expands

A pre-FROM ARG is a global build arg and is not in scope inside any
build stage. With it unset in the stage, ${GIT_VERSION} in the curl
URL expanded to an empty string, so we fetched
github.com/git/git/archive/refs/tags/v.tar.gz, got a 404, and the
piped tar exited 2. BuildKit reports the entire RUN command in its
error, which made it look like make was the failing step.

Verified locally: with `ARG GIT_VERSION` re-declared inside the
stage, the variable expands to 2.54.0 as intended.