fuzzing: refactor OSS-Fuzz build script

[AdamKorcz]

Summary

Parameters to github.com/AdamKorcz/instrumentation are now passed with flags, so update that in Rekors build script.

Remove clone's and wget's out of the build script. I will add these to the OSS-Fuzz Dockerfile which is the suitable place for them, but I need this PR to merge first.

Release Note

Documentation

[codecov]

Codecov Report

Merging #1377 (3185245) into main (8fd34b4) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1377      +/-   ##
==========================================
- Coverage   64.41%   64.39%   -0.03%     
==========================================
  Files          82       82              
  Lines        7908     7908              
==========================================
- Hits         5094     5092       -2     
- Misses       2164     2166       +2     
  Partials      650      650              

Flag	Coverage Δ
e2etests	47.85% <ø> (ø)
unittests	43.00% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/types/rekord/v0.0.1/entry.go	66.97% <0.00%> (-0.63%)	⬇️

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