Don't update build thumbprint's mtime unnecessarily

The build thumbprint changes whenever the build number changes, which
used to be every build. But now local builds don't change the build
number.

The build fingerprint file a few lines above already had this code,
make the thumbprint match.

Bug: 411231354
Test: when building target-files.zip in soong-only builds, repeated builds don't trigger rebuilds
Change-Id: I17c422d2096b58c2c3f38617c6983b87d72c9225