Don't set a default VERS

The default caboose version is currently `0.0.0-git`. This is a mostly
meaningless placeholder. Remove it so we have no version to access
until we have properly signed and versioned a build.

build/xtask/src/dist.rs

@@ -609,7 +609,7 @@ pub fn package(
                 // The Git hash is included in the default caboose under the key
                 // `GITC`, so we don't include it in the pseudo-version.
                 archive
-                    .write_default_caboose(Some(&"0.0.0-git".to_owned()))
+                    .write_default_caboose(None)
                     .context("writing caboose into archive")?;
                 archive.overwrite().context("overwriting archive")?;
             }