Revert "Merge pull request #11300 from noamraph/homeless-shelter-to-proc"

This reverts commit 43e82c9446, reversing
changes made to d79b9bdec0.

Since /proc/homeless-shelter returns a different errno than /homeless-shelter (ENOENT vs EACCES), we need to revert this change.
Software depends on this error code i.e. cargo and therefore breaks.

doc/manual/src/language/derivations.md

@@ -264,8 +264,7 @@ The [`builder`](#attr-builder) is executed as follows:
   - `PATH` is set to `/path-not-set` to prevent shells from
     initialising it to their built-in default value.
 
-  - `HOME` is set to `/proc/homeless-shelter` on Linux and `/homeless-shelter`
-    on OSX, to prevent programs from
+  - `HOME` is set to `/homeless-shelter` to prevent programs from
     using `/etc/passwd` or the like to find the user's home
     directory, which could cause impurity. Usually, when `HOME` is
     set, it is used as the location of the home directory, even if

src/libstore/unix/build/local-derivation-goal.cc

@@ -102,14 +102,7 @@ void handleDiffHook(
     }
 }
 
-// We want $HOME to be un-creatable in the sandbox. On Linux,
-// you can't create anything inside /proc since it's a virtual filesystem.
-// On Darwin it seems that `/homeless-shelter` is good enough.
-#if __linux__
-const Path LocalDerivationGoal::homeDir = "/proc/homeless-shelter";
-#else
 const Path LocalDerivationGoal::homeDir = "/homeless-shelter";
-#endif
 
 
 LocalDerivationGoal::~LocalDerivationGoal()