home-manager: improve robustness

With this commit the activation script is run only if nix-build
succeeded.

home-manager/home-manager

@@ -12,14 +12,14 @@ function doRebuild() {
     local wrkdir
     wrkdir="$(mktemp -d)"
 
-    nix-build --show-trace \
+    if nix-build --show-trace \
                  "@HOME_MANAGER_EXPR_PATH@" \
                  --argstr modulesPath "@MODULES_PATH@" \
                  --argstr confPath "$confFile" \
                  -A activation-script \
-        -o "$wrkdir/generation"
-
+                 -o "$wrkdir/generation" ; then
         "$wrkdir/generation/activate"
+    fi
 
     rm -r "$wrkdir"
 }