util/syspolicy/source: fix data race between Reader.Close and reload

Reader.Close set r.store to nil without holding r.mu, while reload read
r.store while holding r.mu. If a policy store is closed while a
concurrent reload is in flight, reload could observe a nil store and
crash tailscaled with a nil interface method call in
readPolicySettingValue.

Nil out r.store only while holding r.mu, and make reload return the
last known policy once the reader is closing instead of reading from
a store that may no longer exist.

Fixes tailscale/corp#45548
Fixes tailscale/triage#394

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I494cfe9ea1df67b563bc061db9e6944f87b42a4e