util/run: ensure env variable PWD is always correct

Some scripts may rely on $PWD to get current working directory. Since some commands are called after directory is changed by os.chdir(), this variable needs to be manually updated to reflect that change.

---

• The other option would be to call subprocess.Popen(cwd=X) instead, but I saw you use the with cd(...): construct for other stuff as well, so I just did a minimal fix.
• This fixes Debian 9 packaging for knot-resolver (which relies on $PWD)
• It'd be great to get a pip release, since this blocks knot/knot-resolver!1178 (merged)
• integration test pipeline: https://gitlab.nic.cz/packaging/apkg/-/pipelines/83296

apkg/util/run.py

@@ -134,11 +134,15 @@ def cd(newdir):
     Temporarily change current directory.
     """
     olddir = os.getcwd()
-    os.chdir(os.path.expanduser(str(newdir)))
+    oldpwd = os.environ['PWD']
+    newpath = os.path.abspath(os.path.expanduser(str(newdir)))
+    os.chdir(newpath)
+    os.environ['PWD'] = newpath
     try:
         yield
     finally:
         os.chdir(olddir)
+        os.environ['PWD'] = oldpwd
 
 
 class ShellCommand: