Improve performance of entropy build action (#1407)

* Use multiple threads for entropy builds

* Verify builds parameter

* Revert "Verify builds parameter"

This reverts commit 460d3d3b55.

* Use options instead

* Seed fix

* 256 samples on push

tools/multi-build.ps1

@@ -0,0 +1,56 @@
+if ($args.count -lt 1) {
+    Write-Error "Requires 1 arg: number of builds for this job."
+    exit 1
+}
+
+$BuildCount = [int]$args[0]
+
+$build_ids = 0..($BuildCount - 1)
+$build_dirs   = foreach($i in $build_ids) { "build$i" }
+$stdout_files = foreach($i in $build_ids) { "stdout$i.txt" }
+$stderr_files = foreach($i in $build_ids) { "stderr$i.txt" }
+
+# Create unique temp dir for each build thread
+$temp_dirs = foreach($dir in $build_dirs) { "$env:temp\$dir" }
+New-Item -ItemType Directory -Force -Path $temp_dirs
+
+$procs = New-Object System.Collections.Generic.List[System.Diagnostics.Process]
+
+foreach($i in $build_ids) {
+    $params = @{
+        FilePath = "cmake"
+        PassThru = $null
+        ArgumentList = @("--build", $build_dirs[$i])
+        Environment = @{ TEMP = $temp_dirs[$i]; TMP = $temp_dirs[$i] }
+    }
+
+    # For the first job, display stdout and stderr.
+    # Else dump to file so we don't see 50 at once.
+    if ($i -eq 0) {
+        $params.Add("NoNewWindow", $null)
+    } else {
+        $params.Add("RedirectStandardOutput", $stdout_files[$i])
+        $params.Add("RedirectStandardError", $stderr_files[$i])
+    }
+
+    $procs.Add($(Start-Process @params))
+}
+
+
+$failed = $false
+
+# Wait for all builds to finish
+try { Wait-Process -InputObject $procs } catch { $failed = $true }
+
+# Check for failure
+foreach($i in $build_ids) {
+    if ($procs[$i].ExitCode -ne 0) {
+        if ($i -ne 0) {
+            Get-Content $stdout_files[$i] -Tail 10
+            Get-Content $stderr_files[$i] -Tail 10
+        }
+        $failed = $true
+    }
+}
+
+if ($failed) { exit 1 }