Always provide the same date and time in 'Py_GetBuildInfo'.
This is the information shown at the REPL and in 'sys.version'.
We cannot pass it in CPPFLAGS due to whitespace in the DATE string.

--- a/Modules/getbuildinfo.c
+++ b/Modules/getbuildinfo.c
@@ -4,6 +4,10 @@
 #include <stdio.h>
 #endif
 
+/* Deterministic date and time.  */
+#define DATE "Jan  1 1970"
+#define TIME "00:00:01"
+
 #ifndef DATE
 #ifdef __DATE__
 #define DATE __DATE__
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<tr><td><span title='2023-12-06 23:50:04 +0100'>2023-12-06</span></td><td><a href='/guix/commit/tests/guix-shell.sh?id=d98a0203b733316a8a5c9440469dfdd10fc9613f'>shell: ‘--development’ honors ‘--system’.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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