Remove amalg.janet dependency on os/date
When compiling Janet with `JANET_REDUCED_OS`, `os/date` will not be available which breaks the tool amalg.janet. One can check file modification time on the filesystem instead.
# Creates an amalgamated janet.c
# Head
-(def {:year YY :month MM :month-day DD} (os/date))
(print "/* Amalgamated build - DO NOT EDIT */")
-(print "/* Generated " YY "-" (inc MM) "-" (inc DD)
- " with janet version " janet/version "-" janet/build " */")
+(print "/* Generated from janet version " janet/version "-" janet/build " */")
(print "#define JANET_BUILD \"" janet/build "\"")
(print ```#define JANET_AMALG```)
(print ```#include "janet.h"```)
