BUILD.gn previously hard-coded read_file(".git/packed-refs", ...) and
".git/HEAD" to derive electron_version. In a `git worktree` checkout
.git is a file containing a gitdir: pointer, not a directory, so GN's
read_file() fails and gn gen aborts unless override_electron_version is
set manually.
Ask git itself for the real locations via `git rev-parse --git-dir` /
`--git-common-dir` in a small helper script, and feed those resolved
paths to read_file() and the exec_script dependency list. Behaviour in
a plain clone is unchanged (both resolve to electron/.git/...), and the
tarball case still fails loudly with a pointer to
override_electron_version.
