Trouble with initdb trying to run regression tests

I've built Postgres inside a Ubuntu Vagrant VM. When I try to "make check",
I get a complaint about the permissions on the data directory:

[....]
pg_regress: initdb failed
Examine /vagrant/src/test/regress/log/initdb.log for the reason.
Command was: "initdb" -D "/vagrant/src/test/regress/./tmp_check/data"
--no-clean --no-sync > "/vagrant/src/test/regress/log/initdb.log" 2>&1
make[1]: *** [GNUmakefile:125: check] Error 2
make[1]: Leaving directory '/vagrant/src/test/regress'
make: *** [GNUmakefile:69: check] Error 2
vagrant(at)ubuntu-focal:/vagrant$ tail /vagrant/src/test/regress/log/initdb.log
creating subdirectories ... ok
selecting dynamic shared memory implementation ... posix
selecting default max_connections ... 20
selecting default shared_buffers ... 400kB
selecting default time zone ... Etc/UTC
creating configuration files ... ok
running bootstrap script ... 2021-03-30 21:38:32.746 UTC [23154] FATAL:
data directory "/vagrant/src/test/regress/./tmp_check/data" has invalid
permissions
2021-03-30 21:38:32.746 UTC [23154] DETAIL: Permissions should be u=rwx
(0700) or u=rwx,g=rx (0750).
child process exited with exit code 1
initdb: data directory "/vagrant/src/test/regress/./tmp_check/data" not
removed at user's request
vagrant(at)ubuntu-focal:/vagrant$

Has anybody had this problem? The directory in question is created by the
make check activities so I would have thought that it would set the
permissions; and if not, then everybody trying to run regression tests
would bump into this.