just run fails in libfuzzer_libpng_launcher due to binary naming mismatch

[kd610]

Describe the bug
In fuzzers/inprocess/libfuzzer_libpng_launcher/Justfile, there is a naming mismatch between the fuzzer build target and the run/test targets:

• The fuzzer target (lines 58, 64) outputs the binary as fuzzer_libpng_launcher (without .coverage suffix).
• The run target (line 73) expects fuzzer_libpng_launcher.coverage (with .coverage suffix).
• The test target (lines 85, 96, 109) expects fuzzer_libpng_launcher (without suffix).

-> This inconsistency causes just run to fail with "not found" error.

To Reproduce

1. Navigate to fuzzers/inprocess/libfuzzer_libpng_launcher
2. Run just run
3. The error can be seen at the end:
   ./fuzzer_libpng_launcher.coverage --broker-port 21337 --cores 0 --input ./corpus
   sh: 1: ./fuzzer_libpng_launcher.coverage: not found

Expected behavior
just run should successfully build and execute the fuzzer without any not-found errors.