fix: remove initial global HTTP client usage (#20128)

This PR makes the initial steps at removing usage of the global Go HTTP
client, which was seen to have impacts on test flakiness in
https://github.com/coder/internal/issues/1020. The first commit removes
uses from tests, with the exception of one test that is tightly coupled
to the default client. The second commit makes easy/low-risk removals
from application code. This should have some impact to reduce test flakiness.

coderd/healthcheck/accessurl_test.go

@@ -55,7 +55,7 @@ func TestAccessURL(t *testing.T) {
 		defer cancel()
 
 		report.Run(ctx, &healthcheck.AccessURLReportOptions{
-			Client:    nil, // defaults to http.DefaultClient
+			Client:    &http.Client{},
 			AccessURL: nil,
 		})
 

coderd/healthcheck/derphealth/derp_test.go

@@ -511,7 +511,8 @@ func tsDERPMap(ctx context.Context, t testing.TB) *tailcfg.DERPMap {
 	req, err := http.NewRequestWithContext(ctx, "GET", ipn.DefaultControlURL+"/derpmap/default", nil)
 	require.NoError(t, err)
 
-	res, err := http.DefaultClient.Do(req)
+	client := &http.Client{}
+	res, err := client.Do(req)
 	require.NoError(t, err)
 	defer res.Body.Close()
 	require.Equal(t, http.StatusOK, res.StatusCode)