go.toolchain.rev: bump oss, test toolchain matches go.toolchain.rev

Update go.toolchain.rev for https://github.com/tailscale/go/pull/104 and
add a test that, when using the tailscale_go build tag, we use the
right Go toolchain.

We'll crank up the strictness in later commits.

Updates #13527

Change-Id: Ifb09a844858be2beb144a420e4e9dbdc5c03ae3a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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