fix(src/path_env.rs): Issue 2504: Fix for JoinPathsError

[mcallaway]

The original submitter of Issue 2504 triggered the JoinPathsError when adding a python env in the [env] section:

_.python.venv = { path = ".venv", create = true } # create the venv if it doesn't exist

I noticed the same error independently when using the poetry plugin and attempting to run [tasks]. Consider the following .mise.toml file that works fine:

[tools]
[tools.poetry]
version = 'latest'
pyproject = 'pyproject.toml'

[tools.python]
version = '3.11'
# Note this is commented out
#virtualenv = '.venv'

[tasks.foo]
run = "echo foo"

Running the foo task works:

❯ mise r foo
[foo] $ echo foo
foo

But now if I activate that virtualenv config line:

[tools]
[tools.poetry]
version = 'latest'
pyproject = 'pyproject.toml'

[tools.python]
version = '3.11'
virtualenv = '.venv'

[tasks.foo]
run = "echo foo"

Now running the task triggers the same JoinPathsErr:

❯ mise r foo
The application panicked (crashed).
Message:  called `Result::unwrap()` on an `Err` value: JoinPathsError { inner: JoinPathsError }
Location: src/path_env.rs:50

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

In exploring the bug I observed that path_env.add() was being called with two copies of the .venv directory concatenated with a colon ":". So my fix is to update the add() function to check for this, split the path argument on ":" and add each element.

My proposed fix is in ./src/path_env.rs

[mcallaway]

This PR addresses #2504

[jdx]

Some minor feedback but this seems like a solid patch!