write soong.environment.used.* file only if changed

Bug: b/255309129
Test: `lunch aosp_x86-userdebug` and `export NINJA_ARGS="-d explain"`
      Then do the following:
        1) rm -rf out && m bp2build
        2) touch bionic/libc && m bp2build
        3) ALLOW_MISSING_DEPENDENCIES=true m bp2build
      After each step above observe the following two
        a)`stat out/soong/soong.environment.used.bp2build | grep Modify`
        b)`stat out/soong/bp2build_workspace_marker | grep Modify`
      Verified:
       step 2 changes only (b) - prior to this CL, it'd change (a) as well
       step 3 still changes both (as expected)
Change-Id: I98a94878a14e19043f448b0904fc67d5d1dc9733