Store the log file in $TMPDIR instead of /tmp

Instead of using `/tmp` as the base we now use the value of `$TMPDIR`
which is user-scoped. This means that the path of the log file is no
longer `/tmp/pinentry-touchid.log` but `$TMPDIR/pinentry-touchid.log`.

This is particularly important in multiuser environments where the first
user would get ownership of the file and actions from other users would
fail.

Fixes #13

Author: jorgelbg

.github/ISSUE_TEMPLATE/bug_report.md

@@ -45,4 +45,4 @@ Add/attach the relevant section of the log to this issue (feel free to redact yo
 
 **`pinentry-touchid`:**
 
-`pinentry-touchid` also generates its own log which you can find in `/tmp/pinentry-touchid.log`.
+`pinentry-touchid` also generates its own log which you can find in `$TMPDIR/pinentry-touchid.log`.

main.go

@@ -45,10 +45,11 @@ const (
 )
 
 var (
+	// DefaultLogLocation is the location of the log file
+	DefaultLogLocation = filepath.Join(filepath.Clean(os.TempDir()), DefaultLogFilename)
+
 	emailRegex = regexp.MustCompile(`\"(?P<name>.*<(?P<email>.*)>)\"`)
 	keyIDRegex = regexp.MustCompile(`ID (?P<keyId>.*),`) // keyID should be of exactly 8 or 16 characters
-	// DefaultLogLocation is the location of the log file
-	DefaultLogLocation = filepath.Join(filepath.Clean("/tmp"), DefaultLogFilename)
 
 	errEmptyResults    = errors.New("no matching entry was found")
 	errMultipleMatches = errors.New("multiple entries matched the query")