feat: allow templating in env vars (#114)

## Description

This allows templating of env vars in actions.

## Related Issue

Fixes #111 

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [X] New feature (non-breaking change which adds functionality)
- [ ] Other (security config, docs update, etc)

## Checklist before merging

- [X] Test, docs, adr added or updated as needed
- [X] [Contributor Guide
Steps](https://github.com/defenseunicorns/maru-runner/blob/main/CONTRIBUTING.md)
followed

src/pkg/runner/actions.go

@@ -163,6 +163,11 @@ func RunAction[T any](action *types.BaseAction[T], envFilePath string, variableC
 	// Template dir string
 	cfg.Dir = utils.TemplateString(variableConfig.GetSetVariables(), cfg.Dir)
 
+	// Template env strings
+	for idx := range cfg.Env {
+		cfg.Env[idx] = utils.TemplateString(variableConfig.GetSetVariables(), cfg.Env[idx])
+	}
+
 	duration := time.Duration(cfg.MaxTotalSeconds) * time.Second
 	timeout := time.After(duration)
 

src/test/e2e/runner_test.go

@@ -261,6 +261,13 @@ func TestTaskRunner(t *testing.T) {
 		require.Contains(t, stdErr, "SECRET_KEY=not-a-secret")
 	})
 
+	t.Run("test that setting an env var from a variable is processed correctly", func(t *testing.T) {
+		t.Parallel()
+		stdOut, stdErr, err := e2e.Maru("run", "env-templating", "--file", "src/test/tasks/tasks.yaml")
+		require.NoError(t, err, stdOut, stdErr)
+		require.Contains(t, stdErr, "hello-replaced")
+	})
+
 	t.Run("test that env vars get used for variables that do not have a default set", func(t *testing.T) {
 		t.Parallel()
 		os.Setenv("MARU_LOG_LEVEL", "debug")

src/test/tasks/tasks.yaml

@@ -158,3 +158,9 @@ tasks:
     actions:
       - cmd: cat ${COOL_FILE}
         dir: ${COOL_DIR}
+  - name: env-templating
+    description: Tests setting an env var from variable
+    actions:
+      - cmd: echo ${HELLO_KITTEH}
+        env:
+          - HELLO_KITTEH=hello-${REPLACE_ME}