fix(config): When merging, replace rather than combining specific configuration keys

[arlosi]

In general, Cargo merges lists in configuration by concatenating them. However, sometimes the lists don't make sense for merging, such as a program and its arguments. We had the UnmergedStringList type that handled this case for merging environment variables, but it did not work for multiple configs.

• Removes the UnmergedStringList type, which only worked for preventing merging of environment variables with configuration.
• Adds a new function is_nonmergable_list which hard-codes which configuration keys contain lists that should not be merged.

Fixes #14906

[rustbot]

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[epage]

When we talked about this in the Cargo team meeting (2025-01-07), we were ok with this so long as we felt there was a proper way of fixing this later. One idea to do that was to delay merging until access. We're unsure how difficult that would be to implement.

[rustbot]

☔ The latest upstream changes (possibly 149aa21) made this pull request unmergeable. Please resolve the merge conflicts.