guix: fix first run reproducibility defect

contrib/guix/guix-build

@@ -346,7 +346,15 @@ INFO: Building ${VERSION:?not set} for platform triple ${HOST:?not set}:
           ...bind-mounted in container to: '$(OUTDIR_BASE=/outdir-base && outdir_for_host "$HOST")'
 EOF
 
-       rm -f "$(profiledir_for_host "${HOST}")"
+        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'.