Report toolchain xxx is not installed

[XuJiandong]

Describe the bug
Build https://github.com/nervosnetwork/godwoken-scripts with 0.7.2:
❯ capsule --version
Capsule 0.7.2
❯ capsule build
Building contract state-validator
error: toolchain 'nightly-2021-08-16-x86_64-unknown-linux-gnu' is not installed
error: docker container exit with code Some(1)

The capsule.toml file: https://github.com/nervosnetwork/godwoken-scripts/blob/master/capsule.toml#L5
It might be the problem of the following:

[rust]
workspace_dir = "contracts"
toolchain = "nightly-2021-08-16"

The image should be download automatically.

Screenshots
If applicable, add screenshots to help explain your problem. (The command and the output)

Additional context
Please post the following informations:

• Output of capsule --version
• Output of ckb-cli --version
• Rerun your command with RUST_BACKTRACE=1 RUST_LOG=capsule=trace environment, then post the output as a file.

[TheWaWaR]

I think the right way it stick the docker_image version:
godwokenrises/godwoken-scripts#103

[blckngm]

Problem already solved.

FWIW, using cross can help with similar problems in the future: #84