home-manager: simplify use of nix-build output

There is no need to specify an out link when switching to a new
generation since nix-build prints the store path on standard output.
Similarly, when just building a generation we specify no out link
since nix-build will use "result" by default.

(cherry picked from commit 2245b0ac94)

home-manager/home-manager

@@ -49,27 +49,11 @@ function setHomeManagerModulesPath() {
 }
 
 function doBuild() {
-    if [[ -z "$1" ]]; then
-        echo "Need to provide generation output path."
-        exit 1
-    fi
-
-    if [[ -e "$1" ]]; then
-        echo "The output path $1 already exists."
-        exit 1
-    fi
-
     setConfigFile
     setHomeManagerModulesPath
 
-    output="$(realpath "$1")"
-
-    if [[ $? -ne 0 ]]; then
-        exit 1
-    fi
-
     local extraArgs
-    extraArgs=""
+    extraArgs="$1"
 
     for p in "${EXTRA_NIX_PATH[@]}"; do
         extraArgs="$extraArgs -I $p"
@@ -83,21 +67,15 @@ function doBuild() {
               "@HOME_MANAGER_EXPR_PATH@" \
               --argstr confPath "$HOME_MANAGER_CONFIG" \
               --argstr confAttr "$HOME_MANAGER_CONFIG_ATTRIBUTE" \
-              -A activation-script \
-              -o "$output"
+              -A activation-script
 }
 
 function doSwitch() {
-    local wrkdir
-    wrkdir="$(mktemp -d)"
-
     local generation
     local exitCode=0
-    generation=$(doBuild "$wrkdir/result") && $generation/activate || exitCode=1
 
-    # Because the previous command never fails, the script keeps running and
-    # $wrkdir is always removed.
-    rm -r "$wrkdir"
+    generation=$(doBuild "--no-out-link") && $generation/activate || exitCode=1
+
     return $exitCode
 }
 
@@ -179,7 +157,7 @@ cmd="$*"
 
 case "$cmd" in
     build)
-        doBuild "result"
+        doBuild ""
         ;;
     switch)
         doSwitch