container: use /proc/self/exe directly

This is a more reliable form of pathname to self and also cheaper than os.Executable.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/lockedfile/lockedfile_test.go

@@ -15,7 +15,7 @@ import (
 	"testing"
 	"time"
 
-	"hakurei.app/container"
+	"hakurei.app/container/fhs"
 	"hakurei.app/internal/lockedfile"
 	"hakurei.app/internal/lockedfile/internal/testexec"
 )
@@ -215,7 +215,7 @@ func TestSpuriousEDEADLK(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	cmd := testexec.CommandContext(t, t.Context(), container.MustExecutable(nil), "-test.run=^"+t.Name()+"$")
+	cmd := testexec.CommandContext(t, t.Context(), fhs.ProcSelfExe, "-test.run=^"+t.Name()+"$")
 	cmd.Env = append(os.Environ(), fmt.Sprintf("%s=%s", dirVar, dir))
 
 	qDone := make(chan struct{})