ipn/ipnlocal: fix the path for writing cert files (#7203)

Fixes #7202.

Change-Id: I1f8e9c59d5e42e7df7a3fbbd82ae2b4293845916
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/testdata/example.com-key.pem

@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----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+-----END PRIVATE KEY-----