Use DYLD_FALLBACK_LIBRARY_PATH for dylib_path_envvar on macOS

When loading and linking a dynamic library or bundle, dlopen
searches in LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, PWD, and
DYLD_FALLBACK_LIBRARY_PATH.
In the Mach-O format, a dynamic library has an "install path."
Clients linking against the library record this path, and the
dynamic linker, dyld, uses it to locate the library.
dyld searches DYLD_LIBRARY_PATH *before* the install path.
dyld searches DYLD_FALLBACK_LIBRARY_PATH only if it cannot
find the library in the install path.
Setting DYLD_LIBRARY_PATH can easily have unintended
consequences.

See https://users.rust-lang.org/t/subprocess-and-dynamic-library-linking-problem-interaction/7873/10
See https://trac.macports.org/ticket/57692
See https://bugzilla.mozilla.org/show_bug.cgi?id=1536486

This is the same change as was applied to cargo in
rust-lang/cargo#6355

src/toolchain.rs

@@ -388,7 +388,18 @@ impl<'a> Toolchain<'a> {
         }
         #[cfg(target_os = "macos")]
         mod sysenv {
-            pub const LOADER_PATH: &'static str = "DYLD_LIBRARY_PATH";
+            // When loading and linking a dynamic library or bundle, dlopen
+            // searches in LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, PWD, and
+            // DYLD_FALLBACK_LIBRARY_PATH.
+            // In the Mach-O format, a dynamic library has an "install path."
+            // Clients linking against the library record this path, and the
+            // dynamic linker, dyld, uses it to locate the library.
+            // dyld searches DYLD_LIBRARY_PATH *before* the install path.
+            // dyld searches DYLD_FALLBACK_LIBRARY_PATH only if it cannot
+            // find the library in the install path.
+            // Setting DYLD_LIBRARY_PATH can easily have unintended
+            // consequences.
+            pub const LOADER_PATH: &'static str = "DYLD_FALLBACK_LIBRARY_PATH";
         }
         env_var::prepend_path(sysenv::LOADER_PATH, vec![new_path.clone()], cmd);