Fix one of upstream non-determinism, tracked here:

https://github.com/JuliaLang/julia/issues/34115
https://github.com/JuliaLang/julia/issues/25900


Patch by Nicoló Balzarotti <nicolo@nixo.xyz>.

--- a/base/loading.jl
+++ b/base/loading.jl
@@ -807,7 +807,10 @@
         path = normpath(joinpath(dirname(prev), _path))
     end
     if _track_dependencies[]
-        push!(_require_dependencies, (mod, path, mtime(path)))
+          push!(_require_dependencies,
+                (mod, path,
+                 haskey(ENV, "SOURCE_DATE_EPOCH") ?
+                 parse(Float64, ENV["SOURCE_DATE_EPOCH"]) : mtime(path)))
     end
     return path, prev
 end
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<tr><td><span title='2023-04-21 16:16:38 +0200'>2023-04-21</span></td><td><a href='/guix/commit/tests/guix-daemon.sh?id=37dd69b44511dc73eb04bdebe8d82c9a0386338e'>tests: Fix checks for expected failures.</a><span class='msg-avail'>...</span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2022-10-22 01:46:55 +0200'>2022-10-22</span></td><td><a href='/guix/commit/tests/guix-daemon.sh?id=a3619079f95213c4f983e69210ed12b38fd31022'>Remove now unnecessary uses of (guix grafts).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>