Panic when search_dirs.path doesn't exist

[ohlus]

After updating, I started getting panics on some of my machines every time I tried to run tms.

The cause is that some directories in my config were missing on the filesystem. Env vars expansion introduced in #73 unwraps the results of shellexpand and canonicalize. And canonicalize fails when the path doesn't exist.

$ RUST_BACKTRACE=1 tms
thread 'main' panicked at src/configs.rs:142:26:
called `Result::unwrap()` on an `Err` value: IO Error
│
╰─▶ No such file or directory (os error 2)
    ╰╴backtrace (1)

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backtrace no. 1
   0: error_stack::report::Report<C>::from_frame
   1: <core::result::Result<T,C> as error_stack::result::ResultExt>::change_context
   2: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
   3: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
   4: tms::configs::Config::search_dirs
   5: tms::main
   6: std::sys_common::backtrace::__rust_begin_short_backtrace
   7: std::rt::lang_start::{{closure}}
   8: std::panicking::try
   9: std::rt::lang_start_internal
  10: main
  11: __libc_start_call_main
  12: __libc_start_main@@GLIBC_2.34
  13: _start
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
   4: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
   5: tms::configs::Config::search_dirs
   6: tms::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.