avoid hadrian/bindist/Makefile install_docs error when --docs=none
This fixes the regression (5b35ca58) for hadrian --docs=none
that happens for example in 9.4.1-alpha1:
Copying docs to /builddir/build/BUILDROOT/ghc9.4-9.4.0.20220501-10.fc37.x86_64/usr/share/doc/ghc9.4
/usr/bin/install -c -m 755 -d "/builddir/build/BUILDROOT/ghc9.4-9.4.0.20220501-10.fc37.x86_64/usr/share/doc/ghc9.4"
for i in ; do \
cp -R $i "/builddir/build/BUILDROOT/ghc9.4-9.4.0.20220501-10.fc37.x86_64/usr/share/doc/ghc9.4/"; \
done
/usr/bin/install -c -m 644 docs-utils/prologue.txt "/builddir/build/BUILDROOT/ghc9.4-9.4.0.20220501-10.fc37.x86_64/usr/share/doc/ghc9.4/html/libraries/"
/usr/bin/install: cannot stat 'docs-utils/prologue.txt': No such file or directory
make: *** [Makefile:195: install_docs] Error 1
see for example https://kojipkgs.fedoraproject.org/work/tasks/7439/86677439/build.log
Some more detailed comments also in the commit message