`x clean --stage 1` silently does the wrong thing

[jyn514]

I tried this code:

x clean --stage 1

I expected to see this happen: bootstrap only deletes stage 1 artifacts:

; ls build/x86_64-unknown-linux-gnu/stage1* -d
build/x86_64-unknown-linux-gnu/stage1        build/x86_64-unknown-linux-gnu/stage1-tools
build/x86_64-unknown-linux-gnu/stage1-rustc  build/x86_64-unknown-linux-gnu/stage1-tools-bin
build/x86_64-unknown-linux-gnu/stage1-std

Instead, this happened: bootstrap completely ignored the --stage flag and deleted everything in the build directory except for cache.

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