oidc: fix NewProvider ignoring caller context timeout

NewAuthProviderOIDC received a context with a 30-second timeout from
its caller, but immediately discarded it by passing context.Background()
to oidc.NewProvider. This caused both `headscale serve` and
`headscale configtest` to hang indefinitely if the OIDC issuer was
unreachable, rather than timing out and returning an error.

Fix by passing the caller's ctx to oidc.NewProvider and remove the
nolint:contextcheck annotation that was suppressing the linter warning
about this bug. The annotation was introduced in ce580f82 as part of a
bulk lint-suppression pass without addressing the underlying issue.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

hscontrol/oidc.go

@@ -87,7 +87,7 @@ func NewAuthProviderOIDC(
 ) (*AuthProviderOIDC, error) {
 	var err error
 	// grab oidc config if it hasn't been already
-	oidcProvider, err := oidc.NewProvider(context.Background(), cfg.Issuer) //nolint:contextcheck
+	oidcProvider, err := oidc.NewProvider(ctx, cfg.Issuer)
 	if err != nil {
 		return nil, fmt.Errorf("creating OIDC provider from issuer config: %w", err)
 	}