download(): Clear libpath environment variables before invoking curl

We generally don't have a problem when invoking external executables,
however occasionally we are doing something such as running `rr`, which
needs `libgcc_s` from the Julia CSL distribution, and the only way to
get `libgcc_s` for `rr` is to tack its location on to the end of
`LD_LIBRARY_PATH`.  This causes all other executables to search that
directory as well, and right now, that directory is Julia's entire
private libdir, which includes many things such as `libcurl.so`.  This
breaks a system-provided `curl`.

This wil be fixed by JLL stdlib bundling, as we will have much more
fine-grained control over which libraries are added to the paths for
which executables, however in the meantime, an easy fix is to isolate
the system `curl` from our libraries by removing the libpath environment
variables before invoking it.

base/download.jl

@@ -35,9 +35,21 @@ function find_curl()
     end
 end
 
+# Clear libpath for invoking system executables that we generally don't want to be confused
+# by things like a libcurl that sits in Julia's private libdir.
+function clear_libpath(f::Function)
+    withenv(f,
+        "LD_LIBRARY_PATH" => nothing,
+        "DYLD_FALLBACK_LIBRARY_PATH" => nothing,
+        "DYLD_LIBRARY_PATH" => nothing,
+    )
+end
+
 function download_curl(curl_exe::AbstractString, url::AbstractString, filename::AbstractString)
     err = PipeBuffer()
-    process = run(pipeline(`$curl_exe -s -S -g -L -f -o $filename $url`, stderr=err), wait=false)
+    process = clear_libpath() do
+        run(pipeline(`$curl_exe -s -S -g -L -f -o $filename $url`, stderr=err), wait=false)
+    end
     if !success(process)
         error_msg = readline(err)
         @error "Download failed: $error_msg"
@@ -64,14 +76,18 @@ function download(url::AbstractString, filename::AbstractString)
         return download_powershell(url, filename)
     elseif Sys.which("wget") !== nothing
         try
-            run(`wget -O $filename $url`)
+            clear_libpath() do
+                run(`wget -O $filename $url`)
+            end
         catch
             rm(filename, force=true)  # wget always creates a file
             rethrow()
         end
     elseif Sys.which("busybox") !== nothing
         try
-            run(`busybox wget -O $filename $url`)
+            clear_libpath() do
+                run(`busybox wget -O $filename $url`)
+            end
         catch
             rm(filename, force=true)  # wget always creates a file
             rethrow()