Don't use which curl, use Sys.which("curl")

This fixes issues on systems that don't have `which` available

base/download.jl

@@ -4,7 +4,7 @@
 
 downloadcmd = nothing
 if Sys.iswindows()
-    downloadcmd = :powershell
+    downloadcmd = "powershell"
     function download(url::AbstractString, filename::AbstractString)
         ps = "C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\powershell.exe"
         tls12 = "[System.Net.ServicePointManager]::SecurityProtocol = [System.Net.SecurityProtocolType]::Tls12"
@@ -18,23 +18,24 @@ else
     function download(url::AbstractString, filename::AbstractString)
         global downloadcmd
         if downloadcmd === nothing
-            for checkcmd in (:curl, :wget, :fetch)
-                if success(pipeline(`which $checkcmd`, devnull))
+            for checkcmd in ("curl", "wget", "fetch")
+                try
+                    # Sys.which() will throw() if it can't find `checkcmd`
+                    Sys.which(checkcmd)
                     downloadcmd = checkcmd
-                    break
                 end
             end
         end
-        if downloadcmd == :wget
+        if downloadcmd == "wget"
             try
                 run(`wget -O $filename $url`)
             catch
                 rm(filename)  # wget always creates a file
                 rethrow()
             end
-        elseif downloadcmd == :curl
+        elseif downloadcmd == "curl"
             run(`curl -g -L -f -o $filename $url`)
-        elseif downloadcmd == :fetch
+        elseif downloadcmd == "fetch"
             run(`fetch -f $filename $url`)
         else
             error("no download agent available; install curl, wget, or fetch")