Ensure that tar sets file modified time correctly

For some reason, when the source dist archive is built, the file
modified times can be in the future. By adding the `--touch` argument,
all files have their modified time set to the extraction time. This
prevents the "infinite loop make" behavior described in
#14440

These changes also update the arguments passed to `tar` to use full
arguments that are easier to understand, as well as removes the need
for the `xzcat` command, as all build enviroments should have `xz`
support built into GNU `tar`.

Author: lukebakken

packaging/generic-unix/Makefile

@@ -38,7 +38,7 @@ dist:
 	@elixir --version
 	@echo '--------------------------------------------------'
 	@echo
-	xzcat $(SOURCE_DIST_FILE) | tar -xf -
+	tar --touch --xz --extract --file $(SOURCE_DIST_FILE)
 
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