We previously baked in the LetsEncrypt x509 root CA for our tlsdial
package.
This moves that out into a new "bakedroots" package and is now also
shared by ipn/ipnlocal's cert validation code (validCertPEM) that
decides whether it's time to fetch a new cert.
Otherwise, a machine without LetsEncrypt roots locally in its system
roots is unable to use tailscale cert/serve and fetch certs.
Fixes#14690
Change-Id: Ic88b3bdaabe25d56b9ff07ada56a27e3f11d7159
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
150cd30b1d