Set the time zone in main and run Entry in its own test process (closes #192)

cli.Entry() set time.Local to UTC and ended with memguard.Purge(), and
TestEntryWipesBuffersAndReturnsExitCode ran it in the same process as
the other tests. A timer left by an earlier test could read time.Local
as it changed, which the race detector reported on some runs, and the
purge could destroy a buffer still in use, the likeliest cause of a
crash seen on another run.

main() in cmd/secret now sets time.Local before it calls cli.Entry();
the test that runs the built binary with TZ=America/New_York still
covers it. The Entry test runs cli.Entry() in a copy of the test binary
started with SECRET_TEST_RUN_ENTRY set, which init picks up so that
TestMain does not run there.

Model: opus-5-5

cmd/secret/main.go

@@ -3,10 +3,16 @@ package main
 
 import (
 	"os"
+	"time"
 
 	"sneak.berlin/go/secret/internal/cli"
 )
 
 func main() {
+	// Every time secret prints or writes is in UTC, whatever TZ or the
+	// system's time zone is set to. It is set here, before cli.Entry starts
+	// any goroutine that could read time.Local while it changes.
+	time.Local = time.UTC //nolint:gosmopolitan // this makes local time UTC
+
 	os.Exit(cli.Entry())
 }