guix: fix first run reproducibility defect

contrib/guix/guix-build

@@ -348,6 +348,14 @@ EOF
 
         rm -f "$(profiledir_for_host "${HOST}")"
 
+        # First run produces a different GUIX_ENVIRONMENT.
+        time-machine environment --manifest="${PWD}/contrib/guix/manifest.scm" \
+                                 --container \
+                                 --pure \
+                                 --no-cwd \
+                                 -- env HOST="$HOST"
+
+
         # Run the build script 'contrib/guix/libexec/build.sh' in the build
         # container specified by 'contrib/guix/manifest.scm'.
         #