Allow setting working directory for exec.Run

Fixes #536

Closes #537
cuelang/cue#537

GitOrigin-RevId: 5bb80f0ec131e8d1c9cdbe9abdb37429bb1f8359
Change-Id: I4d93e6930c82ea29454b3c53c898b60d70155be6
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9922
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/testdata/script/cmd_dir.txt

@@ -0,0 +1,28 @@
+cue cmd ls
+cmp stdout expect-stdout
+
+-- expect-stdout --
+a.txt
+b.txt
+
+-- ls-work-dir/a.txt --
+-- ls-work-dir/b.txt --
+
+-- ls_tool.cue --
+package ls
+
+command: ls: {
+    ls: {
+		kind:   "exec"
+		cmd:    "ls"
+		dir:    "ls-work-dir"
+		stdout: string
+	}
+
+	task: display: {
+		kind: "print"
+		text: ls.stdout
+	}
+}
+
+-- cue.mod --

pkg/tool/exec/exec.cue

@@ -21,6 +21,10 @@ Run: {
 	// cmd is the command to run.
 	cmd: string | [string, ...string]
 
+	// dir specifies the working directory of the command.
+	// The default is the current working directory.
+	dir?: string
+
 	// env defines the environment variables to use for this system.
 	// If the value is a list, the entries mus be of the form key=value,
 	// where the last value takes precendence in the case of multiple

pkg/tool/exec/exec.go

@@ -137,6 +137,8 @@ func mkCommand(ctx *task.Context) (c *exec.Cmd, doc string, err error) {
 
 	cmd := exec.CommandContext(ctx.Context, bin, args...)
 
+	cmd.Dir, _ = ctx.Obj.Lookup("dir").String()
+
 	env := ctx.Obj.Lookup("env")
 
 	// List case.