fixed symbolic link dereference when launching shell

Without this if we pty.spaw with cwd beeing a symbolic link,
then actually shell will be opened with cwd dereferenced.

src/unixTerminal.ts

@@ -64,6 +64,7 @@ export class UnixTerminal extends Terminal {
     }
 
     const cwd = opt.cwd || process.cwd();
+    env.PWD = cwd
     const name = opt.name || env.TERM || DEFAULT_NAME;
     env.TERM = name;
     const parsedEnv = this._parseEnv(env);