Always copy PHP and legacy CLI files if they have changed

[pjcdawkins]

This checks the existing PHP and legacy CLI files on every run, and re-copies them if necessary.

On Linux it compares the file content (chunk by chunk).

On Windows it compares against a known hash of the Zip file, and if that has changed, it unzips the file again. It also checks that hash during build against the remote sha256sum.txt file, to confirm that the curl download succeeded.

Original description:

Saves static hashes for each file and checks their size / hash on each run

Suggestion for #92

[pjcdawkins]

@akalipetis this will conflict with the vendorization work - but I was playing with it anyway, to find an efficient way to check the files

[akalipetis]

The main issues in here are:

1. There's no need to include a new library (afero) for abstracting the filesystem, we can use fs.FS instead
2. The hash should be computed from the actual file, not from a file containing the hash value - this beats the purpose of checking hashes in the first place

[pjcdawkins]

Re.

There's no need to include a new library (afero) for abstracting the filesystem, we can use fs.FS instead

fs.FS is read only, and ideally we would test writing files, which afero would support.

But, we could drop all of the testable FS stuff and just write real temporary files with os.