fix(linux): use $XDG_RUNTIME_DIR on GNU/Linux

Some systems mount `/tmp` with `noexec`, causing `spawn EACCES` errors.

See: balena-io/etcher#1699
See: https://specifications.freedesktop.org/basedir-spec/latest/ar01s03.html
Change-Type: patch
Changelog-Entry: Store GNU/Linux temporary scripts in `$XDG_RUNTIME_DIR`.
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

lib/execute.js

@@ -24,6 +24,20 @@ const path = require('path');
 const os = require('os');
 const crypto = require('crypto');
 
+/**
+ * @summary A temporary directory to store scripts
+ * @type {String}
+ * @constant
+ *
+ * @description
+ * Some users mount their system temporary directory with the `noexec`
+ * option, which means we can't execute the files we put there.
+ *
+ * As a workaround, we try to use XDG_RUNTIME_DIR, which is probably
+ * a better bet.
+ */
+const TMP_DIRECTORY = process.env.XDG_RUNTIME_DIR || os.tmpdir();
+
 /**
  * @summary Generate a tmp filename with full path of OS' tmp dir
  * @function
@@ -38,7 +52,7 @@ const crypto = require('crypto');
 const tmpFilename = (extension) => {
   const random = crypto.randomBytes(6).toString('hex');
   const filename = `${packageInfo.name}-${random}${extension}`;
-  return path.join(os.tmpdir(), filename);
+  return path.join(TMP_DIRECTORY, filename);
 };
 
 /**