oss-fuzz/build.sh: move flags to 'set' from #!

[jzern]

The set command is preferred over flags in the shebang because they
won't be used if the script is invoked with bash as is done in the
build_fuzzers command 1.

[jzern]

This brings out some other issues with the script and its interaction with the build_fuzzers command. I missed them because there's link errors with --sanitizer undefined which prompted this. --sanitizer address proceeds further, but fails with:

Fuzz binaries: ./tests/avif_fuzztest_dec
./tests/avif_fuzztest_dec_incr
./tests/avif_fuzztest_enc_dec
./tests/avif_fuzztest_enc_dec_anim
./tests/avif_fuzztest_enc_dec_incr
./tests/avif_fuzztest_read_image
./tests/avif_fuzztest_yuvrgb
WARNING: TEST_DATA_DIRS env variable not set, unable to read seed files
WARNING: TEST_DATA_DIRS env variable not set, unable to read seed files
WARNING: TEST_DATA_DIRS env variable not set, unable to read seed files
mkdir: cannot create directory '/out/corpus': File exists
ERROR:__main__:Building fuzzers failed.

[jzern]

This brings out some other issues with the script and its interaction with the build_fuzzers command. I missed them because there's link errors with --sanitizer undefined which prompted this. --sanitizer address proceeds further, but fails with:

Fuzz binaries: ./tests/avif_fuzztest_dec
./tests/avif_fuzztest_dec_incr
./tests/avif_fuzztest_enc_dec
./tests/avif_fuzztest_enc_dec_anim
./tests/avif_fuzztest_enc_dec_incr
./tests/avif_fuzztest_read_image
./tests/avif_fuzztest_yuvrgb
WARNING: TEST_DATA_DIRS env variable not set, unable to read seed files
WARNING: TEST_DATA_DIRS env variable not set, unable to read seed files
WARNING: TEST_DATA_DIRS env variable not set, unable to read seed files
mkdir: cannot create directory '/out/corpus': File exists
ERROR:__main__:Building fuzzers failed.

I had a dirty build directory, though for convenience -p could be used.

The real issue is with cp:

...
cp: -r not specified; omitting directory '/src/libavif/tests/data/goldens'
cp: -r not specified; omitting directory '/src/libavif/tests/data/io'
cp: -r not specified; omitting directory '/src/libavif/tests/data/sources'