pkg/pinned: Update the test TLS certificate
pkg/pinned: Update the test TLS certificate

Go no longer supports 512 bit RSA keys, so use the default size of 2048
bits.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

flynn/flynn,flynn/flynn,lmars/flynn,flynn/flynn,lmars/flynn,lmars/flynn,lmars/flynn,lmars/flynn,flynn/flynn,flynn/flynn