Make the tests fast under the race detector (closes #120)

Deriving keys from passphrases with scrypt, slow on purpose, took most
of the test time under -race. secret.ScryptWorkFactor, when not zero,
replaces age's work factor when a passphrase encrypts; the tests of
internal/secret, internal/vault and internal/cli set it to 1 in
TestMain, and the program never sets it. Decryption reads the factor
from the encrypted data.

TestRemovalAsksWithoutHoldingLock and TestFailedCommandReleasesLock
wait at most 10 seconds for the in-memory lock all tests share, so they
no longer run in parallel with the tests that hold it.

The script/cibuild comment no longer says tests are skipped without
its memlock ulimit.

Model: opus-5-5

script/cibuild

@@ -1,9 +1,9 @@
 #!/bin/sh
 # script/cibuild: run the CI build. The Dockerfile runs script/check
 # (via make check), so a successful build implies all checks pass.
-# The Gitea workflow runs this on push. The memlock ulimit lets the tests
-# that lock large secrets in memory (memguard mlocks them) run; under the
-# lower limit of a plain `docker build .` they are skipped.
+# The Gitea workflow runs this on push. The memlock ulimit lifts the limit
+# on memory the tests lock (memguard mlocks secrets); they also pass under
+# the lower limit of a plain `docker build .`.
 # A cached build checks nothing: a new CHECK_EPOCH on every run makes the
 # Dockerfile's check steps run again on an unchanged tree, while its base
 # images and module downloads stay cached.