Fix bug with PathAndArg config values

[Jake-Shadle]

This fixes an issue I noticed when trying to specify a target runner via the CARGO_TARGET_{triplet}_RUNNER environment variable, expecting it to override the value in our .cargo/config.toml file, which was giving quite strange errors until I figured out that cargo was actually merging the config file's values with the environment's values.

This change adds a small hack to use and UnmergedStringList from PathAndArgs instead of just plain StringList, which uses the type in the deserializer to determine if Config::get_list_or_string should merge the values from the config file(s) with the environment as it was doing before, or else only use the environment to the exclusion of the config file(s) if the key was set in the environment.

I also added a test for this to confirm both the bug and the fix.

[rust-highfive]

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[ehuss]

Thanks! For context, this was changed in #7891 (in 1.43), and the change to runner wasn't intended (not sure how I missed that).

@bors r+

[bors]

📌 Commit ddc7090 has been approved by ehuss

[bors]

⌛ Testing commit ddc7090 with merge f03698b...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing f03698b to master...

[Jake-Shadle]

No worries, thanks for the quick merge!