~/.config/awesome: Don't assume that xkbswap is in path.

I need this because I am currently autostarting the X server on startup
and thus PATH is not set prior to starting awesome.

.config/awesome/rc.lua

                  awful.util.getdir("cache") .. "/history_eval")
              end),
    awful.key({ modkey }, "l", function() os.execute("xkbswap") end),
    awful.key({ modkey }, "l", function() os.execute(os.getenv('HOME') .. "/.bin/xkbswap") end),
    awful.key({ modkey, "Shift" }, "l", function() os.execute("xlock -mode blank") end),
    awful.key({ modkey }, "b", function() awful.util.spawn("chromium --allow-file-access-from-files", false) end),
    awful.key({ modkey, "Shift" }, "b", function() awful.util.spawn("chromium --incognito --allow-file-access-from-files", false) end)