bootspec: do not build two many json object at once

bootspec: do not build two many json object at once

This is a workaround for an issue in the memory sanitizer.
If a function is called with too many arguments, then the sanitizer
triggers the following false-positive warning:

==349==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7f8b247134a7 in json_buildv /work/build/../../src/systemd/src/shared/json.c:3213:17
    #1 0x7f8b24714231 in json_build /work/build/../../src/systemd/src/shared/json.c:4117:13
    #2 0x7f8b24487fa5 in show_boot_entries /work/build/../../src/systemd/src/shared/bootspec.c:1424:29
    #3 0x4a6a1b in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/fuzz/fuzz-bootspec.c:119:16
    #4 0x4c6693 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #5 0x4c5e7a in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:514:3
    #6 0x4c7ce4 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:826:7
    #7 0x4c7f19 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:857:3
    #8 0x4b757f in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    #9 0x4e0bd2 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #10 0x7f8b23ead082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)
    #11 0x41f69d in _start (build-out/fuzz-bootspec+0x41f69d)

Follow-up for #24541.
Fixes #24551.