BUG #16750: VACUUM sporadically fails to access pgstat on Cygwin

The following bug has been logged on the website:

Bug reference:      16750
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 12.5
Operating system:   Windows (Cygwin)
Description:

As the buildfarm shows ([1], [2], [3]), VACUUM (and CREATE INDEX) from time
to time can produce the following warnings (that leads to test failures):

https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=lorikeet&dt=2020-09-07%2020%3A31%3A16&stg=install-check-C
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=lorikeet&dt=2019-10-27%2009%3A36%3A07

pg_regress/vacuum WARNING:  could not open statistics file
"/cygdrive/w/lorikeet/REL_12_STABLE/global.stat": Permission denied
pg_regress/create_index WARNING:  could not open statistics file
"/cygdrive/w/lorikeet/REL_13_STABLE/global.stat": Device or resource busy

The issue with the "Permission denied" error is already resolved for Windows
(see bug #16161), but cygwin still suffers from it.

[1]
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=lorikeet&dt=2020-06-04%2003%3A51%3A45
[2]

https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=lorikeet&dt=2020-08-17%2008%3A28%3A25&stg=install-check-C
[3]
https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=lorikeet&dt=2020-09-02%2008%3A28%3A53&stg=install-check-C