vendor: add z/OS build flags to vendored afero

[msradam]

What?

Add zos build tags to vendored spf13/afero files to enable building on z/OS USS. This mirrors the upstream fix in spf13/afero#384 which was merged in v1.15.0, but the vendored version (v1.1.2) predates it.

• const_bsds.go: add zos to include list (uses syscall.EBADF)
• const_win_unix.go: add !zos to exclude list (syscall.EBADFD unavailable)

Why?

I am actively promoting the use of the k6 test tool in our z/OS test squads. However, it is difficult to maintain the most up-to-date version due to build complexities with cloning both afero and k6. I wanted to streamline it by allowing a simpler build that just involves a git clone and go build. These changes fix the build error:

  vendor/github.com/spf13/afero/const_win_unix.go:25:23: undefined: syscall.EBADFD

Follow-up to #2888 and #2892. Closes #5547

# ./k6 version
k6 v1.5.1-0.20260109162857-b9b1d57948fc+dirty (commit/b9b1d57948-dirty, go1.25.3, zos/s390x)
# k6

         /\      Grafana   /‾‾/  
    /\  /  \     |\  __   /  /   
   /  \/    \    | |/ /  /   ‾‾\ 
  /          \   |   (  |  (‾)  |
 / __________ \  |_|\_\  \_____/ 

Full CLI documentation is available at: https://grafana.com/docs/k6/latest/using-k6/k6-options/reference/

Usage:
  k6 [command]

Available Commands:
  archive     Create an archive
  cloud       Run a test on the cloud
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  inspect     Inspect a script or archive
  new         Create and initialize a new k6 script
  pause       Pause a running test
  resume      Resume a paused test
  run         Start a test
  scale       Scale a running test
  stats       Show test metrics
  status      Show test status
  version     Show application version

Flags:
  -a, --address string              address for the REST API server (default "localhost:6565")
  -c, --config string               JSON config file (default "/ist/home/amrahman/.config/k6/config.json")
  -h, --help                        help for k6
      --log-format string           log output format
      --log-output string           change the output for k6 logs, possible values are: 'stderr', 'stdout',
                                    'none', 'loki[=host:port]', 'file[=./path.fileformat]' (default "stderr")
      --no-color                    disable colored output
      --profiling-enabled           enable profiling (pprof) endpoints, k6's REST API should be enabled as well
  -q, --quiet                       disable progress updates
      --secret-source stringArray   setting secret sources for k6 file[=./path.fileformat],
  -v, --verbose                     enable verbose logging
      --version                     version for k6

Use "k6 [command] --help" for more information about a command.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

No new tests added, this is a build-tag-only change. The change mirrors upstream spf13/afero#384 and the past modification to ui_unix.go.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

[inancgumus]

Hi, @msradam, why do you think we get a build error at the linter stage?