tweak CI

[cdisselkoen]

Additional confidence that each test in the matrix is running with the correct corresponding LLVM version

[Benjins]

Looked into the LLVM 11 error a bit: it looks like the following is happening:

• The latest version of LLVM 11.x installed from apt is 11.1.0 (using llvm-config-11 --version)
• The llvm-11 feature is dependent on llvm-sys version 110.0.4, which links against LLVM version 11.0.x

• The strict version feature flags require that the LLVM major and minor versions of LLVM both match, so 11.0 and 11.1 are considered incompatible

  Unfortunately, I didn't see a 111.x version of llvm-sys which would fix this

  https://github.com/cdisselkoen/llvm-ir/blob/7f81d607f484b2e4fea874ccf818c146ee3d8b56/Cargo.toml#L18

[cdisselkoen]

@Benjins Thanks for investigating that!