selftests/powerpc: Use date instead of EPOCHSECONDS in mitigation-patching.sh

The EPOCHSECONDS environment variable was added in bash 5.0 (released
2019).  Some distributions of the "stable" and "long-term" variety ship
older versions of bash than this, so swap to using the date command
instead.

"%s" was added to coreutils `date` in 1993 so we should be good, but who
knows, it is a GNU extension and not part of the POSIX spec for `date`.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211025102436.19177-1-ruscur@russell.cc