967b724e75 ("build: avoid dynamic dates for reproducibility")
introduced the `mu_date` variable to hard-code a build-date. This is
used for the dates embedded in the texinfo documentation, but `date` is
still called to set the copyright years in the man-pages. Use `mu_date`
there too.
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
c9b06a55e7