Use read to read data from a FileHandle instead of availableData (#486)

Analogous fix to swiftlang/swift-package-manager#8047

Author: ahoppen

Sources/TSCBasic/Process/Process.swift

@@ -575,7 +575,7 @@ public final class Process {
 
             group.enter()
             stdoutPipe.fileHandleForReading.readabilityHandler = { (fh : FileHandle) -> Void in
-                let data = fh.availableData
+                let data = (try? fh.read(upToCount: Int.max)) ?? Data()
                 if (data.count == 0) {
                     stdoutPipe.fileHandleForReading.readabilityHandler = nil
                     group.leave()
@@ -590,7 +590,7 @@ public final class Process {
 
             group.enter()
             stderrPipe.fileHandleForReading.readabilityHandler = { (fh : FileHandle) -> Void in
-                let data = fh.availableData
+                let data = (try? fh.read(upToCount: Int.max)) ?? Data()
                 if (data.count == 0) {
                     stderrPipe.fileHandleForReading.readabilityHandler = nil
                     group.leave()