All packages are still built with rustc 1.37, but users will see rustc
1.38 in their profiles, and packages that require newer compiler
versions will be possible to add.
* gnu/packages/rust.scm (rust-1.31): Factor out the
patch-command-exec-tests phase to a separate procedure.
(patch-command-exec-tests-phase): New procedure.
(rust): Rename to rust-1.37, make rust an alias.
(rust-1.38): New variable.
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