Revert "Update the config crate from 0.13.4 to 0.14.0"

This reverts commit 2ec9497.

We do unfortunately have to downgrade since `config 0.14.0` [0] converts
all configuration keys to lowercase but some of our configuration keys,
e.g. the environment variables, are case sensitive.
More details are in the relevant upstream PR [1] that made the crate
case insensitive by converting all the keys to lowercase "internally"
and a new issue discussing this change/regression [2].
Unfortunately this wasn't really mentioned as a breaking change in the
upstream config-rs changelog so I missed it.
This fixes such errors:
```
Error: build command failed

Caused by:
    0: Checking allowed variable names
    1: Checking allowed variables for package tree 1.8.0
    2: Environment variable name not allowed: additional_make_flags
```

I didn't notice this regression at first as I relied on the tests but
they don't catch such errors yet. I'll try to add a test case for this
error and check the environment variables defined in `pkg.toml` files
during the loading of the repository (should yield better error messages
and we do want to verify **all** packages and during the loading of the
repository ("evaluation")).

[0]: https://github.com/mehcode/config-rs/blob/0.14.0/CHANGELOG.md#0140---2024-02-01
[1]: rust-cli/config-rs#354
[2]: rust-cli/config-rs#531

Cargo.toml

@@ -32,7 +32,7 @@ chrono = "0.4"
 clap = { version = "4", features = ["cargo"] }
 clap_complete = "4"
 colored = "2"
-config = { version = "0.14", default-features = false, features = [ "toml" ] }
+config = { version = "0.13", default-features = false, features = [ "toml" ] }
 csv = "1"
 daggy = { version = "0.8", features = [ "serde" ] }
 dialoguer = "0.11"