refactor(cmd_exec): resolve sequtils.delete deprecation warning

[ee7]

Like https://github.com/crashappsec/chalk/pull/280, a small PR to get rid of deprecation warnings and reduce noise locally (e.g. when running nimble build -d:debug).

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Resolve the deprecation warning:

src/commands/cmd_exec.nim(214, 16) Warning: use delete(s, first..last); delete is deprecated [Deprecated]

Nim deprecated the previous form in July 2021 (see commit, source, and changelog) for consistency with changes to system.delete, which previously had surprising behavior when the given index was out of bounds.