build_dist.sh: use $go consistently.

The invocation at the end unconditionally used
./tool/go, but the structuring on lines 14-17
sets up to use a different toolchain if the
platform requires it.

Fixes #8156

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

build_dist.sh

@@ -49,4 +49,4 @@ while [ "$#" -gt 1 ]; do
 	esac
 done
 
-exec ./tool/go build ${tags:+-tags=$tags} -ldflags "$ldflags" "$@"
+exec $go build ${tags:+-tags=$tags} -ldflags "$ldflags" "$@"