rpm: Use bash for %() expansion (jsc#SLE-18234).
    
    Since 15.4 alternatives for /bin/sh are provided by packages
    <something>-sh. While the interpreter for the build script can be
    selected the interpreter for %() cannot.
    
    The kernel spec files use bashisms in %().
    
    While this could technically be fixed there is more serious underlying
    problem: neither bash nor any of the alternatives are 100% POSIX
    compliant nor bug-free.
    
    It is not my intent to maintain bug compatibility with any number of
    shells for shell scripts embedded in the kernel spec file. The spec file
    syntax is not documented so embedding the shell script in it causes some
    unspecified transformation to be applied to it. That means that
    ultimately any changes must be tested by building the kernel, n times if
    n shells are supported.
    
    To reduce maintenance effort require that bash is used for kernel build
    always.
    
    Signed-off-by: Michal Suchanek <msuchanek@suse.de>