Enable disabling --cfg fuzzing_repro

Changing `RUSTFLAGS` causes a recompile of the entire project so for
projects that are expensive to build this option being enabled by
default means that workflows which switch back-and-forth between fuzzing
and running individual tests generate a full rebuild every time. This
adds a `--no-cfg-fuzzing` option on the `cargo fuzz run` CLI to disable
this `--cfg` argument from being passed.

Author: alexcrichton

src/options/run.rs

@@ -29,6 +29,12 @@ pub struct Run {
     /// Number of concurrent jobs to run
     pub jobs: u16,
 
+    /// By default the 'cfg(fuzzing_repro)' compilation configuration is set
+    /// when a single test case is being run, and this flag allows you to opt
+    /// out of it.
+    #[arg(long)]
+    pub no_cfg_fuzzing_repro: bool,
+
     #[arg(last(true))]
     /// Additional libFuzzer arguments passed through to the binary
     pub args: Vec<String>,
@@ -37,7 +43,7 @@ pub struct Run {
 impl RunCommand for Run {
     fn run_command(&mut self) -> Result<()> {
         let project = FuzzProject::new(self.fuzz_dir_wrapper.fuzz_dir.to_owned())?;
-        self.build.cfg_fuzzing_repro = !self.corpus.is_empty();
+        self.build.cfg_fuzzing_repro = !self.corpus.is_empty() && !self.no_cfg_fuzzing_repro;
         project.exec_fuzz(self)
     }
 }