Install cargo only when it is missing

[imobachgs]

Problem

If you install your Rust toolchain using rustup, running the setup-service.sh will fail with the following error:

Problem: the installed rustup-1.25.2~0-4.1.x86_64 conflicts with 'rust+cargo' provided by the to be installed cargo-1.69.0-1.1.x86_64
 Solution 1: deinstallation of rustup-1.25.2~0-4.1.x86_64
 Solution 2: do not install cargo-1.69.0-1.1.x86_64

Solution

Do not install cargo unless it is missing.

[coveralls]

Pull Request Test Coverage Report for Build 5197970484

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.686%

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Totals
Change from base Build 5173879017:	0.0%
Covered Lines:	5396
Relevant Lines:	6890

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💛 - Coveralls