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='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/gnu-pw-mgr.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-11-17 04:45:29 +0100'>2020-11-17</span></td><td><a href='/guix/commit/gnu/packages/gnu-pw-mgr.scm?id=f3effa5132ee9c026d3a0422900f451d4a99fbf3'>gnu: gnu-pw-mgr: Update to 2.7.4.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-20 22:34:53 +0300'>2020-09-20</span></td><td><a href='/guix/commit/gnu/packages/gnu-pw-mgr.scm?id=ff7bba573ac6b51836718cfdc81b638caf11488b'>gnu: gnu-pw-mgr: Update to 2.7.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-03-25 16:02:53 +0100'>2020-03-25</span></td><td><a href='/guix/commit/gnu/packages/gnu-pw-mgr.scm?id=98e9ed8a3fa2841dc24030828baf1fa899a329ef'>gnu: gnu-pw-mgr: Build reproducibly.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>