Fix TODO on clippy false positive warning

[edmundoferreira]

Currently github check like https://github.com/mozilla/grcov/actions/runs/4345920963/jobs/7591298097

error: manual implementation of `Option::filter`
   --> src/main.rs:479:14
    |
479 |           _ => match opt.output_path.as_deref() {
    |  ______________^
480 | |             Some(output_path) => {
481 | |                 if output_path.is_dir() {
482 | |                     Some(output_path)
...   |
487 | |             _ => None,
488 | |         },
    | |_________^ help: try this: `opt.output_path.as_deref().filter(|&output_path| output_path.is_dir())`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_filter
    = note: `-D clippy::manual-filter` implied by `-D warnings`

error: could not compile `grcov` due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `grcov` due to previous error

However the clippy suggestion with .filter(|&output_path| output_path.is_dir()) will not produce the desired result since currently

 _ => match opt.output_path.as_deref() {
            Some(output_path) => {
                if output_path.is_dir() {
                    Some(output_path)
                } else {
                    panic!("output_path must be a directory when using multiple outputs");
                }
            }
            _ => None,
        },

has else code path need to generate a panic!() if the output path is not a directory.

This clippy behaviour has been fixed on rust-lang/rust-clippy#10091 but unfortunately it's not yet released.

As a temporary work around we ignore the clippy warning by adding #[allow(clippy::manual_filter)] to the code block.
This should be removed once the upstream fix to clippy is released
And remove TODO in Originally posted by @marco-c in #979 (comment)