Make the subproject dir src again

We got rid of this in c7ec33605e because
of bug https://github.com/mesonbuild/meson/issues/13774, but in the
previous commit we renamed the manual source directory, which avoids it.
Now we can change it back.

meson.build

@@ -3,6 +3,7 @@
 
 project('nix-dev-shell', 'cpp',
   version : files('.version'),
+  subproject_dir : 'src',
   default_options : [
     'localstatedir=/nix/var',
   ]

subprojects

@@ -1 +0,0 @@
-src