Make the install directory configurable

[me-and]

Default to maintaining the current behaviour, but allow users of this
action to change the install directory. This means it's possible, for
example, to use the default-everywhere-else behaviour of installing
64-bit Cygwin to C:\cygwin64, or to ease migration from other GitHub
Actions such as egor-tensin/setup-cygwin, which defaults to installing
in C:\tools\cygwin.

[me-and]

I tried adding a test, too, but apparently the YAML is invalid:

The workflow is not valid. .github/workflows/test.yml (Line: 119, Col: 14): Unrecognized named-value: 'matrix'. Located at position 1 within expression: matrix.install-dir

I'm utterly failing to work out what the problem is, though – as best I can tell the bit it's objecting to is more-or-less identical to the similar example in "complex-test"…

[me-and]

Ah, fixed that test problem; I was mangling my Git commands unhelpfully.

[me-and]

Okay, that was clearly not ready to be a PR at the time I created it, given just how many times I needed to force-push to correct the test code. But the test is now working on my fork, so I think this PR is actually ready to be looked at and pulled now.

[jon-turney]

Thanks! This was just me being lazy since a fixed path was all I needed for my particular application.

This also needs an change to README.md to document the new parameter.

[me-and]

Okay, removed the extraneous shell declaration in the test script, and updated the README to document the new option.

[jon-turney]

Thanks!