sigstore: wire tests to explicit cosign binaries

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

src/cosign/cosign.ts

@@ -38,7 +38,7 @@ export interface CosignCommandError {
 }
 
 export class Cosign {
-  private readonly binPath: string;
+  public readonly binPath: string;
   private _version: string;
   private _versionOnce: boolean;