build_dist.sh: use cmd/mkversion to get version data

Replaces the former shell goop, which was a shell reimplementation
of a subset of version/mkversion.

Signed-off-by: David Anderson <danderson@tailscale.com>

.github/workflows/test.yml

@@ -55,6 +55,7 @@ jobs:
         GOARCH: ${{ matrix.goarch }}
     - name: build variant CLIs
       run: |
+        export TS_USE_TOOLCHAIN=1
         ./build_dist.sh --extra-small ./cmd/tailscaled
         ./build_dist.sh --box ./cmd/tailscaled
         ./build_dist.sh --extra-small --box ./cmd/tailscaled