fix: clap cannot downcast CLI flags to PathBuf

[jvanz]

Description

After the recent changes clap is not able to cast the CLI flags into PathBuf type. To fix that, we can configure the value parser when the Arg is created.

This is causing errors when running CI on Kubewarden controller main branch and kubewarden/kubewarden-controller#1007

KUBEWARDEN_CERT_FILE="tls.crt" KUBEWARDEN_KEY_FILE="tls.key" KUBEWARDEN_POLICIES_DOWNLOAD_DIR=/tmp KUBEWARDEN_POLICIES=policies.yml.example KUBEWARDEN_LOG_LEVEL=debug KUBEWARDEN_SIGSTORE_CACHE_DIR="/tmp/sigstore-data" KUBEWARDEN_ALWAYS_ACCEPT_ADMISSION_REVIEWS_ON_NAMESPACE="kubewarden" KUBEWARDEN_CLIENT_CA_FILE="ca.crt" cargo run
thread 'main' panicked at src/config.rs:193:29:
Mismatch between definition and access of `cert-file`. Could not downcast to std::path::PathBuf, need to downcast to alloc::string::String

stack backtrace:
   0: rust_begin_unwind
             at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/panicking.rs:74:14
   2: clap_builder::parser::error::MatchesError::unwrap
             at /home/jvanz/.cargo/registry/src/index.crates.io-6f17d22bba15001f/clap_builder-4.5.30/src/parser/error.rs:32:9
   3: clap_builder::parser::matches::arg_matches::ArgMatches::get_one
             at /home/jvanz/.cargo/registry/src/index.crates.io-6f17d22bba15001f/clap_builder-4.5.30/src/parser/matches/arg_matches.rs:119:9
   4: policy_server::config::build_tls_config
             at ./src/config.rs:193:21
   5: policy_server::config::Config::from_args

Test

KUBEWARDEN_CERT_FILE="tls.crt" KUBEWARDEN_KEY_FILE="tls.key" KUBEWARDEN_POLICIES_DOWNLOAD_DIR=/tmp KUBEWARDEN_POLICIES=policies.yml.example KUBEWARDEN_LOG_LEVEL=debug KUBEWARDEN_SIGSTORE_CACHE_DIR="/tmp/sigstore-data" KUBEWARDEN_ALWAYS_ACCEPT_ADMISSION_REVIEWS_ON_NAMESPACE="kubewarden" KUBEWARDEN_CLIENT_CA_FILE="ca.crt" cargo run

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.57%. Comparing base (aecaeba) to head (5f206a6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1096      +/-   ##
==========================================
- Coverage   59.81%   59.57%   -0.25%     
==========================================
  Files          18       18              
  Lines        1217     1217              
==========================================
- Hits          728      725       -3     
- Misses        489      492       +3     

Flag	Coverage Δ
integration-tests	51.03% <ø> (-0.25%)	⬇️
unit-tests	35.47% <ø> (ø)

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[fabriziosestito]

LGTM, thanks for fixing this