cmd/run: Don't check the stdin and stdout in a loop in the fallback case

The outcome of checking whether the standard input and output of the
current invocation of toolbox are connected to a terminal device is
going to stay constant for the life cycle of the process.  So, checking
it repeatedly in a loop when falling back to a different command or
working directory is wasteful.

Secondly, it prevents secondary logic like this from intermingling with
the code that actually assembles the list of arguments.  This makes it
easier to get a quick gist of the final command and its structure.

Fallout from a22d7821cb

src/cmd/run.go

@@ -295,6 +295,18 @@ func runCommandWithFallbacks(container string, command []string, emitEscapeSeque
 
 	envOptions := utils.GetEnvOptionsForPreservedVariables()
 
+	var ttyNeeded bool
+
+	stdinFd := os.Stdin.Fd()
+	stdinFdInt := int(stdinFd)
+
+	stdoutFd := os.Stdout.Fd()
+	stdoutFdInt := int(stdoutFd)
+
+	if term.IsTerminal(stdinFdInt) && term.IsTerminal(stdoutFdInt) {
+		ttyNeeded = true
+	}
+
 	runFallbackCommandsIndex := 0
 	runFallbackWorkDirsIndex := 0
 	workDir := workingDirectory
@@ -305,6 +317,7 @@ func runCommandWithFallbacks(container string, command []string, emitEscapeSeque
 			detachKeysSupported,
 			envOptions,
 			fallbackToBash,
+			ttyNeeded,
 			workDir)
 
 		if emitEscapeSequence {
@@ -444,6 +457,7 @@ func constructExecArgs(container string,
 	detachKeysSupported bool,
 	envOptions []string,
 	fallbackToBash bool,
+	ttyNeeded bool,
 	workDir string) []string {
 	var detachKeys []string
 
@@ -460,13 +474,7 @@ func constructExecArgs(container string,
 
 	execArgs = append(execArgs, detachKeys...)
 
-	stdinFd := os.Stdin.Fd()
-	stdinFdInt := int(stdinFd)
-
-	stdoutFd := os.Stdout.Fd()
-	stdoutFdInt := int(stdoutFd)
-
-	if term.IsTerminal(stdinFdInt) && term.IsTerminal(stdoutFdInt) {
+	if ttyNeeded {
 		execArgs = append(execArgs, "--tty")
 	}