Use a temporary path when testing archive commands.

Previously, fpm would write to the current directory when using methods to check for the correct `ar` and `tar` commands.

This now uses Stud::Temporary to get a random path in the system temporary directory.

I found this error in the test suite when the fpm git workspace was not
writable -- for example, when mounted read-only through a container.

Error message, for posterity:

    1) FPM::Command -p | --package when given a directory should write the package to the given directory.
       Failure/Error: cmd.run(["-s", "empty", "-t", "deb", "-n", "example", "-p", path])
       Errno::EACCES:
         Permission denied @ rb_sysopen - fpm-dummy.tmp

lib/fpm/util.rb

@@ -1,5 +1,6 @@
 require "fpm/namespace"
 require "fileutils"
+require "stud/temporary"
 
 # Some utility functions
 module FPM::Util
@@ -229,15 +230,18 @@ def ar_cmd
     @@ar_cmd_deterministic = false
 
     # FIXME: don't assume current directory writeable
-    FileUtils.touch(["fpm-dummy.tmp"])
+    emptyfile = Stud::Temporary.pathname
+    testarchive = Stud::Temporary.pathname
+    FileUtils.touch([emptyfile])
+
     ["ar", "gar"].each do |ar|
       ["-qc", "-qcD"].each do |ar_create_opts|
-        FileUtils.rm_f(["fpm-dummy.ar.tmp"])
+        FileUtils.rm_f([testarchive])
         # Return this combination if it creates archives without uids or timestamps.
         # Exitstatus will be nonzero if the archive can't be created,
         # or its table of contents doesn't match the regular expression.
         # Be extra-careful about locale and timezone when matching output.
-        system("#{ar} #{ar_create_opts} fpm-dummy.ar.tmp fpm-dummy.tmp 2>/dev/null && env TZ=UTC LANG=C LC_TIME=C #{ar} -tv fpm-dummy.ar.tmp | grep '0/0.*1970' > /dev/null 2>&1")
+        system("#{ar} #{ar_create_opts} #{testarchive} #{emptyfile} 2>/dev/null && env TZ=UTC LANG=C LC_TIME=C #{ar} -tv #{testarchive} | grep '0/0.*1970' > /dev/null 2>&1")
         if $?.exitstatus == 0
            @@ar_cmd = [ar, ar_create_opts]
            @@ar_cmd_deterministic = true
@@ -247,10 +251,8 @@ def ar_cmd
     end
     # If no combination of ar and options omits timestamps, fall back to default.
     @@ar_cmd = ["ar", "-qc"]
+    FileUtils.rm_f([testarchive, emptyfile])
     return @@ar_cmd
-  ensure
-    # Clean up
-    FileUtils.rm_f(["fpm-dummy.ar.tmp", "fpm-dummy.tmp"])
   end # def ar_cmd
 
   # Return whether the command returned by ar_cmd can create deterministic archives
@@ -264,7 +266,10 @@ def tar_cmd
     return @@tar_cmd if defined? @@tar_cmd
 
     # FIXME: don't assume current directory writeable
-    FileUtils.touch(["fpm-dummy.tmp"])
+    emptyfile = Stud::Temporary.pathname
+    testarchive = Stud::Temporary.pathname
+
+    FileUtils.touch([emptyfile])
 
     # Prefer tar that supports more of the features we want, stop if we find tar of our dreams
     best="tar"
@@ -276,7 +281,7 @@ def tar_cmd
       opts=[]
       score=0
       ["--sort=name", "--mtime=@0"].each do |opt|
-        system("#{tar} #{opt} -cf fpm-dummy.tar.tmp fpm-dummy.tmp > /dev/null 2>&1")
+        system("#{tar} #{opt} -cf #{testarchive} #{emptyfile} > /dev/null 2>&1")
         if $?.exitstatus == 0
           opts << opt
           score += 1
@@ -292,10 +297,9 @@ def tar_cmd
       end
     end
     @@tar_cmd = best
+    FileUtils.rm_f([testarchive, emptyfile])
+
     return @@tar_cmd
-  ensure
-    # Clean up
-    FileUtils.rm_f(["fpm-dummy.tar.tmp", "fpm-dummy.tmp"])
   end # def tar_cmd
 
   # Return whether the command returned by tar_cmd can create deterministic archives