net/bakedroots: add LetsEncrypt ISRG Root X2

Updates #14690

Change-Id: Ib85e318d48450fc6534f7b0c1d4cc4335de7c0ff
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/bakedroots/bakedroots.go

@@ -16,7 +16,12 @@ import (
 //
 // As of 2025-01-21, this includes only the LetsEncrypt ISRG Root X1 root.
 func Get() *x509.CertPool {
-	roots.once.Do(func() { roots.parsePEM([]byte(letsEncryptX1)) })
+	roots.once.Do(func() {
+		roots.parsePEM(append(
+			[]byte(letsEncryptX1),
+			letsEncryptX2...,
+		))
+	})
 	return roots.p
 }
 
@@ -120,3 +125,25 @@ mRGunUHBcnWEvgJBQl9nJEiU0Zsnvgc/ubhPgXRR4Xq37Z0j4r7g1SgEEzwxA57d
 emyPxgcYxn/eR44/KJ4EBs+lVDR3veyJm+kXQ99b21/+jh5Xos1AnX5iItreGCc=
 -----END CERTIFICATE-----
 `
+
+// letsEncryptX2 is the ISRG Root X2.
+//
+// Subject: O = Internet Security Research Group, CN = ISRG Root X2
+// Key type: ECDSA P-384
+// Validity: until 2035-09-04 (generated 2020-09-04)
+const letsEncryptX2 = `
+-----BEGIN CERTIFICATE-----
+MIICGzCCAaGgAwIBAgIQQdKd0XLq7qeAwSxs6S+HUjAKBggqhkjOPQQDAzBPMQsw
+CQYDVQQGEwJVUzEpMCcGA1UEChMgSW50ZXJuZXQgU2VjdXJpdHkgUmVzZWFyY2gg
+R3JvdXAxFTATBgNVBAMTDElTUkcgUm9vdCBYMjAeFw0yMDA5MDQwMDAwMDBaFw00
+MDA5MTcxNjAwMDBaME8xCzAJBgNVBAYTAlVTMSkwJwYDVQQKEyBJbnRlcm5ldCBT
+ZWN1cml0eSBSZXNlYXJjaCBHcm91cDEVMBMGA1UEAxMMSVNSRyBSb290IFgyMHYw
+EAYHKoZIzj0CAQYFK4EEACIDYgAEzZvVn4CDCuwJSvMWSj5cz3es3mcFDR0HttwW
++1qLFNvicWDEukWVEYmO6gbf9yoWHKS5xcUy4APgHoIYOIvXRdgKam7mAHf7AlF9
+ItgKbppbd9/w+kHsOdx1ymgHDB/qo0IwQDAOBgNVHQ8BAf8EBAMCAQYwDwYDVR0T
+AQH/BAUwAwEB/zAdBgNVHQ4EFgQUfEKWrt5LSDv6kviejM9ti6lyN5UwCgYIKoZI
+zj0EAwMDaAAwZQIwe3lORlCEwkSHRhtFcP9Ymd70/aTSVaYgLXTWNLxBo1BfASdW
+tL4ndQavEi51mI38AjEAi/V3bNTIZargCyzuFJ0nN6T5U6VR5CmD1/iQMVtCnwr1
+/q4AaOeMSQ+2b1tbFfLn
+-----END CERTIFICATE-----
+`