Updating the action to have a choice for how user is looked up on linux

[sponte]

This is to help with the issue: nodejs/node#25714

/usr/bin/docker exec  94bb9493d85b41dbcf3e7fb97eb2b5bfbe97d4d34d83c8f470c63ce42314ad0c sh -c "cat /etc/*release | grep ^ID"
os.js:272
    throw new ERR_SYSTEM_ERROR(ctx);
    ^
SystemError [ERR_SYSTEM_ERROR]: A system error occurred: uv_os_get_passwd returned ENOENT (no such file or directory)
    at Object.userInfo (os.js:272:11)
    at Object.972 (/__w/_actions/webfactory/ssh-agent/v0.5.4/dist/index.js:579:14)
    at __webpack_require__ (/__w/_actions/webfactory/ssh-agent/v0.5.4/dist/index.js:[22](https://github.com/bank-of-england-technology/lv-3-aks/runs/7083235574?check_suite_focus=true#step:4:23):30)
    at Object.104 (/__w/_actions/webfactory/ssh-agent/v0.5.4/dist/index.js:122:36)
    at __webpack_require__ (/__w/_actions/webfactory/ssh-agent/v0.5.4/dist/index.js:22:30)
    at startup (/__w/_actions/webfactory/ssh-agent/v0.5.4/dist/index.js:37:19)
    at /__w/_actions/webfactory/ssh-agent/v0.5.4/dist/index.js:41:18
    at Object.<anonymous> (/__w/_actions/webfactory/ssh-agent/v0.5.4/dist/index.js:44:10)
    at Module._compile (internal/modules/cjs/loader.js:999:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:10[27](https://github.com/bank-of-england-technology/lv-3-aks/runs/7083235574?check_suite_focus=true#step:4:28):10) {
  code: 'ERR_SYSTEM_ERROR',
  info: {
    errno: -2,
    code: 'ENOENT',
    message: 'no such file or directory',
    syscall: 'uv_os_get_passwd'
  },
  errno: [Getter/Setter],
  syscall: [Getter/Setter]
}