fix: allow source to be used with process substitution

[reubeno]

This fixes issues reported when doing something like source <(some command).

[github-actions]

Test Results

 24 files  144 suites   59s ⏱️
425 tests 413 ✅ 12 💤 0 ❌
854 runs  826 ✅ 28 💤 0 ❌

Results for commit 165844b.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.62 μs	3.35 μs	-0.27 μs	🟢 -7.39%
instantiate_shell	59.76 μs	59.85 μs	0.09 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	25931.69 μs	25936.76 μs	5.08 μs	⚪ Unchanged
parse_bash_completion	5625.13 μs	5613.90 μs	-11.23 μs	⚪ Unchanged
parse_sample_script	9.03 μs	9.15 μs	0.12 μs	🟠 +1.27%
run_echo_builtin_command	92.29 μs	91.14 μs	-1.15 μs	⚪ Unchanged
run_one_builtin_command	112.84 μs	113.43 μs	0.59 μs	⚪ Unchanged
run_one_external_command	1852.06 μs	1874.25 μs	22.19 μs	⚪ Unchanged
run_one_external_command_directly	957.59 μs	961.83 μs	4.24 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/jobs.rs	🔴 42.42%	🔴 37.23%	🔴 -5.19%
brush-core/src/openfiles.rs	🔴 47.71%	🟠 50%	🟢 2.29%
brush-core/src/shell.rs	🟢 78.21%	🟢 78.13%	🔴 -0.08%
Overall Coverage	🟢 76.09%	🟢 76%	🔴 -0.09%

Minimum allowed coverage is 70%, this run produced 76%