nixos/java: No bashisms in environment.shellInit script

Running dash (the lightweight Debian Almquist shell) results in

```
source: not found
```

For this line which ends up in `/etc/profile`. Let's use `.` instead, according to [the Ubuntu wiki](https://wiki.ubuntu.com/DashAsBinSh#source) and [`checkbashisms`](https://sourceforge.net/projects/checkbaskisms/):

```
possible bashism in /etc/profile line [...] (should be '.', not 'source'):
test -e /nix/store/[...]/nix-support/setup-hook && source /nix/store/[...]/nix-support/setup-hook
```

(cherry picked from commit 9692e2e)

nixos/modules/programs/java.nix

@@ -64,7 +64,7 @@ in
     environment.systemPackages = [ cfg.package ];
 
     environment.shellInit = ''
-      test -e ${cfg.package}/nix-support/setup-hook && source ${cfg.package}/nix-support/setup-hook
+      test -e ${cfg.package}/nix-support/setup-hook && . ${cfg.package}/nix-support/setup-hook
     '';
 
   };