Work around nextest macro lib compatibility bug

[EliahKagan]

This sets the RUSTUP_WINDOWS_PATH_ADD_BIN environment variable in the multi-platform test job, to work around nextest-rs/nextest#1493 which is keeping the test runner from working on Windows. See also ruffle-rs/ruffle#16342, which gave the idea for this, and various other projects that have made such changes, linked in nextest-rs/nextest#1493.

Running cargo nextest run --all on Windows produces output like:

$ cargo nextest run --all
warning: function `evaluate_target_dir` is never used
 --> gix-prompt\tests\prompt.rs:9:8
  |
9 |     fn evaluate_target_dir() -> String {
  |        ^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `gix-prompt` (test "prompt") generated 1 warning
   Compiling gix-filter v0.11.1 (C:\Users\ek\source\repos\gitoxide\gix-filter)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 2.39s
error: creating test list failed

Caused by:
  for `gix-macros`, command `'C:\Users\ek\source\repos\gitoxide\target\debug\deps\gix_macros-9b54cd09b507c731.exe' --list --format terse` exited with code 0xc0000135: The specified module could not be found. (os error 126)
--- stdout:

--- stderr:

---

I believe the relevant portion of the output starts at error: creating test list failed. Attempting to run the command manually shows:

$ 'C:\Users\ek\source\repos\gitoxide\target\debug\deps\gix_macros-9b54cd09b507c731.exe' --list --format terse
C:/Users/ek/source/repos/gitoxide/target/debug/deps/gix_macros-9b54cd09b507c731.exe: error while loading shared libraries: std-49e3d1aefc00cc02.dll: cannot open shared object file: No such file or directory

As mentioned in #1371 (comment), this occurs both locally and on CI and is not specific to changes proposed in #1371, nor is it related to gix-macros changes in bad5b48 (#1363).

• Test rerun at the tip of main in my fork, showing the failure
• Test rerun from an old commit in my fork, showing the failure

I've gone ahead and set that environment variable for all three platforms that have test-fast CI jobs, but not in other jobs. It does not appear to cause problems for other platforms. Once this change is no longer needed, it should probably be reverted, but I think it is useful right now because it should let #1371 and any other forthcoming PRs move forward.

[Byron]

Thanks a million!