chore(ci): Fix nextest / rustup dll issue (vectordotdev#20544)

Workaround for issue in nextest caused by a change in rustup:
nextest-rs/nextest#1493

Fixes:

```
error: creating test list failed

Caused by:
  for `vector-config-macros`, command
  `'C:\a\vector\vector\target\debug\deps\vector_config_macros-4fc11cf764d99dff.exe' --list --format
  terse` exited with code 0xc0000135: The specified module could not be found. (os error 126)
```

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

scripts/environment/bootstrap-windows-2019.ps1

@@ -25,3 +25,6 @@ echo "OPENSSL_SRC_PERL=C:\Strawberry\perl\bin\perl.exe" | Out-File -FilePath $en
 
 # Force the proto-build crate to avoid building the vendored protoc.
 echo "PROTO_NO_VENDOR=1" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
+
+# Workaround for https://github.com/nextest-rs/nextest/issues/1493
+echo "RUSTUP_WINDOWS_PATH_ADD_BIN=1" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append