tests: repair oss-fuzz coverage command (#33304)

The coverage build path was generating go test commands with a bogus
-tags flag that held the coverpkg value, so the run kept failing. I
switched coverbuild to treat the optional argument as an override for
-coverpkg and stopped passing coverpkg from the caller. Now the script
emits a clean go test invocation that should actually succeed.

oss-fuzz.sh

@@ -64,7 +64,7 @@ function compile_fuzzer() {
   go get github.com/holiman/gofuzz-shim/testing
 
 	if [[ $SANITIZER == *coverage* ]]; then
-		coverbuild $path $function $fuzzer $coverpkg
+		coverbuild $path $function $fuzzer
 	else
 	  gofuzz-shim --func $function --package $package -f $file -o $fuzzer.a
 		$CXX $CXXFLAGS $LIB_FUZZING_ENGINE $fuzzer.a -o $OUT/$fuzzer