fix: cli variables should take priority over aaskfile defaults (#2589)

When using `task FOO=bar` with a Taskfile containing
`FOO: '{{.FOO | default "foo"}}'`, the CLI value was being
overwritten by the Taskfile default.

Split the variable merging into two steps:
1. Merge CLI variables (FOO=bar) first so they override Taskfile vars
2. ReverseMerge special variables (CLI_ARGS, CLI_FORCE, etc.) so
   they're available for templating in Taskfile vars

Fixes regression introduced in #2403.

Co-authored-by: Timothy Rule <timothy.rule@gmail.com>

testdata/vars/Taskfile.yml

@@ -49,3 +49,10 @@ tasks:
       - echo "{{.MESSAGE}}"
 
   from-dot-env: echo '{{.DOT_ENV_VAR}}'
+
+  # Test that CLI variables take priority over Taskfile defaults
+  cli-var-priority:
+    vars:
+      CLI_VAR: '{{.CLI_VAR | default "default_value"}}'
+    cmds:
+      - echo '{{.CLI_VAR}}'

testdata/vars/testdata/TestVars-cli-var-priority-default.golden

@@ -0,0 +1 @@
+default_value

testdata/vars/testdata/TestVars-cli-var-priority-override.golden

@@ -0,0 +1 @@
+from_cli