client: defer nobody user lookup so Windows doesn't panic

[tgross]

In #14742 we introduced a cached lookup of the nobody user, which is only ever called on Unixish machines. But the initial caching was being done in an init block, which meant it was being run on Windows as well. This prevents the Nomad agent from starting on Windows.

An alternative fix here would be to have a separate init block for Windows and Unix, but this potentially masks incorrect behavior if we accidentally added a call to the Nobody() method on Windows later. This way we're forced to handle the error in the caller.

---

Before:

C:\opt> C:\opt\nomad.exe agent -config C:\etc\nomad.d"
panic: unable to lookup the nobody user: No mapping between account names and security IDs was done.

goroutine 1 [running]:
github.com/hashicorp/nomad/helper/users.init.0()
        github.com/hashicorp/nomad/helper/users/lookup.go:28 +0xfd

After:

C:\opt> C:\opt\nomad.exe agent -config C:\etc\nomad.d
==> Loaded configuration from C:\etc\nomad.d\base.hcl, C:\etc\nomad.d\client-windows-0.hcl, C:\etc\no
mad.d\client-windows.hcl, C:\etc\nomad.d\consul.hcl, C:\etc\nomad.d\tls.hcl, C:\etc\nomad.d\vault.hcl
==> Starting Nomad agent...
...

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