lock_machine: slightly adjust lease duration

When trying to lock a skylab machine, I got the following error:

```
Lease duration (1440 minutes) cannot exceed 1 day [1440 minutes]
```

Though our lease duration doesn't _exceed_ one day, it's easy enough to
drop it by one minute to make the error go away.

BUG=None
TEST=Ran crosperf

Change-Id: I75c11de1a70e46f75a50c5022f6b4860c0338c1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2177065
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
diff --git a/lock_machine.py b/lock_machine.py
index 10e6049..776fe54 100755
--- a/lock_machine.py
+++ b/lock_machine.py
@@ -57,7 +57,7 @@
 
   # TODO(zhizhouy): lease time may needs to be dynamically adjusted. For now we
   # set it long enough to cover the period to finish nightly rotation tests.
-  LEASE_MINS = 1440
+  LEASE_MINS = 1439
 
   SKYLAB_CREDENTIAL = ('/usr/local/google/home/mobiletc-prebuild'
                        '/sheriff_utils/credentials/skylab'