net/tlsdial: fix TLS cert validation of HTTPS proxies

If you had HTTPS_PROXY=https://some-valid-cert.example.com running a
CONNECT proxy, we should've been able to do a TLS CONNECT request to
e.g. controlplane.tailscale.com:443 through that, and I'm pretty sure
it used to work, but refactorings and lack of integration tests made
it regress.

It probably regressed when we added the baked-in LetsEncrypt root cert
validation fallback code, which was testing against the wrong hostname
(the ultimate one, not the one which we were being asked to validate)

Fixes #16222

Change-Id: If014e395f830e2f87f056f588edacad5c15e91bc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/bakedroots/bakedroots.go

@@ -7,6 +7,7 @@ package bakedroots
 
 import (
 	"crypto/x509"
+	"fmt"
 	"sync"
 
 	"tailscale.com/util/testenv"
@@ -14,7 +15,7 @@ import (
 
 // Get returns the baked-in roots.
 //
-// As of 2025-01-21, this includes only the LetsEncrypt ISRG Root X1 root.
+// As of 2025-01-21, this includes only the LetsEncrypt ISRG Root X1 & X2 roots.
 func Get() *x509.CertPool {
 	roots.once.Do(func() {
 		roots.parsePEM(append(
@@ -56,7 +57,7 @@ type rootsOnce struct {
 func (r *rootsOnce) parsePEM(caPEM []byte) {
 	p := x509.NewCertPool()
 	if !p.AppendCertsFromPEM(caPEM) {
-		panic("bogus PEM")
+		panic(fmt.Sprintf("bogus PEM: %q", caPEM))
 	}
 	r.p = p
 }