CI: use rust-toolchain.toml

[boozook]

Using rust-toolchain in toml format.
Now we shouldn't parse file but trust rustup & gh-action.

Waits fixes in gh-action: issue, PR.

[coveralls]

Pull Request Test Coverage Report for Build 1812623463

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 20 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 40.563%

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Files with Coverage Reduction	New Missed Lines	%
primitives/src/currency.rs	20	18.87%

Totals
Change from base Build 1728357566:	-0.7%
Covered Lines:	95
Relevant Lines:	175

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💛 - Coveralls