[swift_snapshot_tool] Default --workspace to use /tmp/swift_snapshot_tool_workspace_v1 instead of /tmp/workspace.

Now that I have put this into swift's utils, it isn't appropriate for me to just
take /tmp/workspace. The name swift_snapshot_tool_workspace_v1 is a better
name.

Author: gottesmm

utils/swift_snapshot_tool/Sources/swift_snapshot_tool/bisect_toolchains.swift

@@ -18,7 +18,7 @@ struct BisectToolchains: AsyncParsableCommand {
     help: """
       The directory where toolchains should be downloaded to.
       """)
-  var workspace: String = "/tmp/workspace"
+  var workspace: String = "/tmp/swift_snapshot_tool_workspace_v1"
 
   @Option(
     help: """