bump rust toolchain to 1.73.0 (nushell#11445)

This PR bumps the nushell rust toolchain from 1.72.1 to 1.73.0 since
1.75.0 was released recently.

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rust-toolchain.toml

@@ -16,4 +16,4 @@ profile = "default"
 # use in nushell, we may opt to use the bleeding edge stable version of rust.
 # I believe rust is on a 6 week release cycle and nushell is on a 4 week release cycle.
 # So, every two nushell releases, this version number should be bumped by one.
-channel = "1.72.1"
+channel = "1.73.0"