[build] Fix `script/build` when outside of a git repository

Fixes error `usage: link [options] main.o` when building from the
downloaded tarball

script/version

@@ -11,7 +11,7 @@ export GIT_CEILING_DIRECTORIES=${PWD%/*}
 version="$(git describe --tags HEAD 2>/dev/null || true)"
 
 if [ -z "$version" ]; then
-  version="$(grep Version version/version.go | head -1 | cut -d '"' -f2)"
+  version="$(grep 'Version =' version/version.go | head -1 | cut -d '"' -f2)"
   sha="$(git rev-parse --short HEAD 2>/dev/null || true)"
   [ -z "$sha" ] || version="${version}-g${sha}"
 fi