go.toolchain.rev: switch to Go 1.19rc2+
Switch to Go 1.19rc2 in prep for the Go 1.19 GA release on Tuesday.
(We won't be using any Go 1.19 features until then.)
Updates #5210
Change-Id: I94fa0ae8f5645fb7579429668f3970c18d1796d8
Signed-off-by: Brad Fitzpatrick <[email protected]>