fix: forward env to RunCommand when evaluating sh vars (#1869)

* forward env to RunCommand when evaluating sh vars. fixes #1742

* feat: added tests

* fix: test

---------

Co-authored-by: Pete Davison <pd93.uk@outlook.com>

testdata/env/Taskfile.yml

@@ -17,6 +17,7 @@ tasks:
       - task: global
       - task: not-overridden
       - task: multiple_type
+      - task: dynamic
 
   local:
     vars:
@@ -50,3 +51,11 @@ tasks:
   overridden:
     cmds:
       - echo "QUX='$QUX'" > overridden.txt
+
+  dynamic:
+    silent: true
+    vars:
+      DYNAMIC_FOO:
+        sh: echo $FOO
+    cmds:
+      - echo "{{ .DYNAMIC_FOO }}" > dynamic.txt