Set the timestamp of downloaded stage0 files ...

... using server-reported timestamp.
This allows us to track changes to the downloaded artifact more easily
and in a more reproducible manner.

Co-authored-by: Zixing Liu <zixing.liu@canonical.com>

src/bootstrap/bootstrap.py

@@ -109,7 +109,7 @@ def _download(path, url, probably_big, verbose, exception):
                 "-L", # Follow redirect.
                 "-y", "30", "-Y", "10",    # timeout if speed is < 10 bytes/sec for > 30 seconds
                 "--connect-timeout", "30",  # timeout if cannot connect within 30 seconds
-                "--retry", "3", "-Sf", url],
+                "--retry", "3", "-SRf", url],
                 stdout=outfile,    #Implements cli redirect operator '>'
                 verbose=verbose,
                 exception=True, # Will raise RuntimeError on failure