Update cheatsheets

rustup-toolchain

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# rustup toolchain
+
+> Manage Rust toolchains.
+> See `rustup help toolchain` for more information about toolchains.
+> More information: <https://rust-lang.github.io/rustup>.
+
+- Install or update a given toolchain:
+
+`rustup install {{toolchain}}`
+
+- Uninstall a toolchain:
+
+`rustup uninstall {{toolchain}}`
+
+- List installed toolchains:
+
+`rustup list`
+
+- Create a custom toolchain by symlinking to a directory:
+
+`rustup link {{custom_toolchain_name}} {{path/to/directory}}`