fix: omit dirs from executable searches

[reubeno]

We were already filtering on executables, but forgot to filter on being a file. Among other things, this fixed an issue spotted in completion of help topics.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.42 μs	3.41 μs	-0.00 μs	⚪ Unchanged
instantiate_shell	61.90 μs	59.68 μs	-2.22 μs	🟢 -3.58%
instantiate_shell_with_init_scripts	30469.18 μs	30359.30 μs	-109.88 μs	⚪ Unchanged
parse_bash_completion	2791.95 μs	2796.06 μs	4.11 μs	⚪ Unchanged
parse_sample_script	4.26 μs	4.38 μs	0.12 μs	🟠 +2.82%
run_echo_builtin_command	92.01 μs	89.22 μs	-2.79 μs	🟢 -3.04%
run_one_builtin_command	111.03 μs	107.48 μs	-3.55 μs	⚪ Unchanged
run_one_external_command	1949.15 μs	1957.47 μs	8.33 μs	⚪ Unchanged
run_one_external_command_directly	1010.62 μs	1006.56 μs	-4.06 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/getopts.rs	🟢 94.81%	🟢 95.12%	🟢 0.31%
brush-core/src/sys/unix/signal.rs	🟠 72.13%	🟢 75.41%	🟢 3.28%
Overall Coverage	🟢 76.47%	🟢 76.5%	🟢 0.03%

Minimum allowed coverage is 70%, this run produced 76.5%

[github-actions]

Test Results

  2 files    9 suites   1m 5s ⏱️
497 tests 497 ✅ 0 💤 0 ❌
983 runs  983 ✅ 0 💤 0 ❌

Results for commit b401580.