Fix static build when zstd is enabled for libelf

Fix static build when zstd is enabled for libelf

When doing a static build (as with --enable-gcov) the build fails
because the executables aren't linked with -lzstd. Also the
run-debuginfod-query-retry.sh test relies on libdebuginfod.so.1 which
isn't there.

Fix by adding $(zstd_LIBS) to the static libelf lines and using the
debuginfod-find executable itself in the testcase.

    * src/Makefile.am (libelf): When BUILD_STATIC add $(zstd_LIBS)
    * tests/Makefile.am (libelf): Likewise.
    * tests/run-debuginfod-query-retry.sh: Use debuginfod-find instead
      of libdebuginfod.so.1

Signed-off-by: Mark Wielaard <mark@klomp.org>