Move dotnet.cmd/sh helper scripts into the repo root (#672)

* Move dotnet helper scripts to repo root

* Don't swallow InitializeDotNetCli output

* Use temporary file to store sdk location

To avoid a nested process chain in which the inner process then invokes
the host, we instead create a temporary file to store the sdk location.

dotnet.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+source="${BASH_SOURCE[0]}"
+# resolve $SOURCE until the file is no longer a symlink
+while [[ -h $source ]]; do
+  scriptroot="$( cd -P "$( dirname "$source" )" && pwd )"
+  source="$(readlink "$source")"
+
+  # if $source was a relative symlink, we need to resolve it relative to the path where the
+  # symlink file was located
+  [[ $source != /* ]] && source="$scriptroot/$source"
+done
+scriptroot="$( cd -P "$( dirname "$source" )" && pwd )"
+
+# Don't resolve runtime, shared framework, or SDK from other locations to ensure build determinism
+export DOTNET_MULTILEVEL_LOOKUP=0
+
+# Disable first run since we want to control all package sources
+export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
+
+source $scriptroot/eng/common/tools.sh
+
+InitializeDotNetCli true # Install
+__dotnetDir=${_InitializeDotNetCli}
+
+dotnetPath=${__dotnetDir}/dotnet
+${dotnetPath} "$@"