Fix flatpak builds
    
    ...after 19065a8b01585a1aa5f22e38e99fc0c47c597074 "Temporarily move x86 to use
    Zero in order to get a working build":
    
    When building the
    
    >       if ${run_bootstrap} ; then
    
    branch for suffix='' and loop='-main', the second
    
    >           buildjdk ${builddir} $(pwd)/${bootinstalldir}/images/%{jdkimage} "${maketargets}" ${debugbuild} ${link_opt}
    
    uses the JDK (`$(pwd)/${bootinstalldir}/images/%{jdkimage}`) from the installjdk
    on the previous line.  But installjdk does
    
    > 	rm ${imagepath}/lib/tzdb.dat
    > 	ln -s %{_datadir}/javazi-1.8/tzdb.dat ${imagepath}/lib/tzdb.dat
    
    which made that JDK's tzdb.dat link to /app/share/javazi-1.8/tzdb.dat in a
    flatpak build (rather than the usual /usr/share/javazi-1.8/tzdb.dat in a non-
    flatpak build) which is not present at build-time (but will be present at
    runtime in at least the LibreOffice flatpak, which bundles tzdata-java built for
    the flatpak /app prefix).  So using that JDK's compiler during the build kept
    failing due to java.io.FileNotFoundException for its lib/tzdb.dat.
    
    (This was not an issue prior to 19065a8b01585a1aa5f22e38e99fc0c47c597074, as
    installjdk's modification of lib/tzdb.dat used to be done only for the "Final
    setup on the main image" at the very end of the build, not during the build for
    JDKs that are themselves used later during the build.)
    
    The easiest workaround for this issue appears to be to just not bootstrap_build
    in the flatpak case, avoiding the situation that a JDK whose lib/tzdb.dat has
    been modified through installjdk is used during the build.