BUG: runtest.py fails with a KeyError

[BvB93]

Since the merging of #19628 the python ./runtest.py fails with a KeyError.

Based on the traceback it seems that sysconfig.get_path('platlib', expand=False) returns an incorrect formattable string.

Error message:

>>> python .\runtests.py
Traceback (most recent call last):
  File "C:\Users\hardd\Documents\GitHub\numpy\runtests.py", line 695, in <module>
    main(argv=sys.argv[1:])
  File "C:\Users\hardd\Documents\GitHub\numpy\runtests.py", line 192, in main
    site_dir, site_dir_noarch = build_project(args)
  File "C:\Users\hardd\Documents\GitHub\numpy\runtests.py", line 480, in build_project
    site_dir = site_dir_template.format(platbase=dst_dir,
KeyError: 'base'

And the content of the offending site_dir_template formattable string:

>>> print(site_dir_template)
{base}/Lib/site-packages

NumPy/Python version information:

1.22.0.dev0+696.g334428c4d
3.9.4 | packaged by conda-forge | (default, May 10 2021, 22:10:34) [MSC v.1916 64 bit (AMD64)]

[mattip]

I wonder how it passed CI.