feat: gate set-env/add-path and render annotation locations (#1109)

`::set-env::` and `::add-path::` let a step rewrite the environment of every later step from its own output, which the runner honoured silently. They are now refused, as GitHub has done since 2020, and `ACTIONS_ALLOW_UNSECURE_COMMANDS` opts back in per step or job. Support for that variable is new here too, and is the only opt-in, matching GitHub rather than adding a runner config key on top.

Annotations keep their source location: Gitea has no annotation store and its web UI strips command properties, so `::error file=main.go,line=12::msg` is rendered as `::error::main.go:12: msg`.

`DEVELOPMENT.md` writes down the log line encoding rules this relies on.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1109
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: bircni <bircni@icloud.com>

act/runner/run_context.go

@@ -22,6 +22,7 @@ import (
 	"runtime"
 	"slices"
 	"strings"
+	"sync"
 	"time"
 
 	"gitea.com/gitea/runner/act/common"
@@ -83,6 +84,25 @@ type RunContext struct {
 	// failures. Those failures must still make success() false and failure() true for later
 	// main-step if evaluation.
 	jobFailed bool
+	// stepEnv is a copy of the running step's environment, so that workflow commands parsed out
+	// of the container's output can be judged against it. Written by runStepExecutor and read on
+	// the log-writer goroutine, hence unsecureCommandMu, which also guards unsecureCommandErr.
+	stepEnv            map[string]string
+	unsecureCommandErr error // refused ::set-env::/::add-path::, turned into a step failure
+	unsecureCommandMu  sync.Mutex
+}
+
+// setCurrentStepEnv records the environment of the step about to run.
+func (rc *RunContext) setCurrentStepEnv(env map[string]string) {
+	rc.unsecureCommandMu.Lock()
+	defer rc.unsecureCommandMu.Unlock()
+	rc.stepEnv = env
+}
+
+func (rc *RunContext) currentStepEnv() map[string]string {
+	rc.unsecureCommandMu.Lock()
+	defer rc.unsecureCommandMu.Unlock()
+	return rc.stepEnv
 }
 
 // markCancelled flags the job as cancelled so subsequent step `if` evaluations and the