ociregistry/ociauth: pass context to token GET requests

Although POST requests to the token server were correctly
being passed the context, GET requests were not.

Remedy that by using `NewRequestWithContext` instead
of `NewRequest`.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I7084253af14dd9b57e92aea86bc161d6a3aceb49
Reviewed-on: https://review.gerrithub.io/c/cue-labs/oci/+/1184461
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUE porcuepine <cue.porcuepine@gmail.com>

ociregistry/ociauth/auth.go

@@ -397,7 +397,7 @@ func (r *registry) acquireToken(ctx context.Context, scope Scope) (*wireToken, e
 		v.Set("service", service)
 	}
 	u.RawQuery = v.Encode()
-	req, err := http.NewRequest("GET", u.String(), nil)
+	req, err := http.NewRequestWithContext(ctx, "GET", u.String(), nil)
 	if err != nil {
 		return nil, err
 	}