Added fallback for finding ld on MinGW Cygwin

[radarhere]

Suggestion to resolve #4018

The mingw32 job is currently failing with

if self.ld_version >= "2.10.90":
    TypeError: '>=' not supported between instances of 'NoneType' and 'str'

Searching, I found that in 'C:/msys64/mingw32/lib/python3.7\distutils\cygwinccompiler.py', there is a part where it stops searching for a simple 'ld' executable, instead searching for the path specified by gcc.

ld = 'ld'
out = Popen(gcc+' --print-prog-name ld', shell=True, stdout=PIPE).stdout
try:
	ld = test=str(out.read(),encoding='utf-8').strip()

However, I think this path then ends up being /usr/lib/gcc/x86_64-pc-msys/9.1.0/../../../../x86_64-pc-msys/bin/ld.exe

So this PR uses pacman versions of pytest and pytest-cov instead of compiling from source, and adds a fallback when compiling Pillow, trying 'ld' again if the gcc path does not work.

[hugovk]

Thanks!