fix: ignore '$TMPDIR' when it is an unusable directory

A recent audit of Ubuntu’s Rust utilities¹ led me to learn that
`$TMPDIR` is only intended to be used when the directory it points to is
accessible:²

  If applications are written to use temporary or intermediate files,
  they should use the _TMPDIR_ environment variable, if it is set and
  represents an accessible directory, to select the location of
  temporary files.

The effect of this change is that `vimcat --help` with an unusable
`$TMPDIR` set will now fall back to using /tmp instead of trying (and
failing) to use this `$TMPDIR`.

¹ https://discourse.ubuntu.com/t/an-update-on-rust-coreutils/80773
² https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap01.html

Author: Smattr

vimcat/help.c

@@ -41,7 +41,7 @@ int help(void) {
 
   // find temporary storage space
   const char *TMPDIR = getenv("TMPDIR");
-  if (TMPDIR == NULL)
+  if (TMPDIR == NULL || access(TMPDIR, R_OK | W_OK | X_OK) != 0)
     TMPDIR = "/tmp";
 
   // create a temporary path