fix(compiler): create task.dir before running dynamic (sh:) vars

[SAY-5]

Fixes #1001.

Problem

A tasks dir: is documented as being created on demand:

dir — Directory which this task should run. Defaults to the current working directory. If the directory does not exist, Task creates it.

but dynamic variables (sh:) are compiled before the tasks mkdir step runs. So this Taskfile:

test-dir:
  dir: does-not-exist
  cmds:
    - echo {{.TEST}}
  vars:
    TEST:
      sh: cat ../exist.txt

aborts with a cryptic chdir error:

task: Command "cat ../exist.txt" failed: chdir /…/does-not-exist: no such file or directory

even though running the same command inside cmds: works fine, because mkdir runs before runCommand for normal commands but after variable compilation.

Fix

Compiler.HandleDynamicVar now creates dir with 0o755 (the same permissions Executor.mkdir uses at task.go:311) before calling execext.RunCommand. Dynamic variables and cmds now see the same filesystem state, and the documented dir: semantics hold for both.

if dir != "" {
    if _, err := os.Stat(dir); os.IsNotExist(err) {
        if err := os.MkdirAll(dir, 0o755); err != nil {
            return "", fmt.Errorf(...)
        }
    }
}

No behaviour change when the directory already exists; no behaviour change for tasks without sh: vars.

Signed off per DCO.

[trulede]

This is the wrong approach to solve this problem. A non-existent path should cause an error, not just create the directory regardless.

The problem needs to be solved elsewhere in the code base.