Fix string context lost in `linkFarm`

```nix
let pkgs = import ./. {}; in
pkgs.linkFarm "test" [
  { name = "foo"; path = ./README.md; }
]
```

The nix code above generates this result: (`nix build -f test.nix`)

```
lrwxrwxrwx 1 root root 31 Jan  1  1970 /nix/store/98mdz626n99w0467q4r203q154bnnli9-test/foo -> /home/juloo/w/nixpkgs/README.md
```

With this patch, the target file is naturally copied into the store:

```
lrwxrwxrwx 1 root root 53 Jan  1  1970 /nix/store/ndvffnardifqwzbp07g15llav55r5k1m-test/foo -> /nix/store/9rqwjskbbgbflrpyhzi6rak2j7jspr5f-README.md
```

pkgs/build-support/trivial-builders.nix

@@ -477,7 +477,7 @@ rec {
       cd $out
       ${lib.concatMapStrings (x: ''
           mkdir -p "$(dirname ${lib.escapeShellArg x.name})"
-          ln -s ${lib.escapeShellArg x.path} ${lib.escapeShellArg x.name}
+          ln -s ${x.path} ${lib.escapeShellArg x.name}
       '') entries}
     '';