This was found when trying to run the fileset tests on Darwin
(NixOS/nix#9546 (comment)), which mysteriously fail on Darwin:

  test case at lib/fileset/tests.sh:342 failed: toSource { root = "/nix/store/foobar"; fileset = ./.; } should have errored with this regex pattern:

  lib.fileset.toSource: `root` \(/nix/store/foobar\) is a string-like value, but it should be a path instead.
  \s*Paths in strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.

  but this was the actual error:

  error: lib.fileset.toSource: `root` (/nix/store/foobar) is a string-like value, but it should be a path instead.
      Paths in strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.

After dissecting this, I find out that apparently \s works on Linux, but not on Darwin for some reason!

From the bash source code, it looks like <regex.h> with `REG_EXTENDED` is used for all platforms the same,
so there's nothing odd there.

It's almost impossible to know where <regex.h> comes from,
but it looks to be a POSIX thing.

So after digging through the almost impossible to find POSIX specifications
(https://pubs.opengroup.org/onlinepubs/007908799/xbd/re.html#tag_007_003_005),
I can indeed confirm that there's no mention of \s or the like!

_However_, there is a mention of `[[:blank:]]`, so we'll use that instead.

(cherry picked from commit 3429594)