Unable to source common.sh when CDPATH is set

[sgillis]

When executing /speckit.plan I get the error

.specify/scripts/bash/setup-plan.sh: line 28: <project_path>/.specify/scripts/bash
<project_path>/.specify/scripts/bash/common.sh: No such file or directory

This can be reproduced by setting CDPATH variable

[localden]

PR with the fix merged.