Support more Linux Distros for hosting the build

- Use `dotnet-install.sh` to get the CLI instead constructing download
  URLs by hand.

- Use a newer version of the CLI when building (this is the same version
  that CoreFX and CoreCLR use).

- Change how BaseNuGetRuntimeIdentifier is computed when running
  on *NIX. We now use the information from /etc/os-release (which is
  what the CLI does internally).

With these changes, I've been able to build Roslyn on RHEL by updating
the runtimes sections of the relevent project.jsons to include
"rhel.7.2-x64".

Makefile

@@ -10,18 +10,16 @@ RESTORE_SEMAPHORE_PATH = $(BINARIES_PATH)/restore.semaphore
 BOOTSTRAP_PATH = $(BINARIES_PATH)/Bootstrap
 BUILD_LOG_PATH =
 HOME_DIR = $(shell cd ~ && pwd)
-DOTNET_VERSION = 1.0.0-preview2-003131
+DOTNET_VERSION = 1.0.0-preview3-003223
 NUGET_VERSION = 3.5.0-beta2
 NUGET_EXE = $(shell pwd)/nuget.exe
 
 MSBUILD_ADDITIONALARGS := /v:m /fl /fileloggerparameters:Verbosity=normal /p:Configuration=$(BUILD_CONFIGURATION)
 
 ifeq ($(OS_NAME),Linux)
-	MSBUILD_ADDITIONALARGS := $(MSBUILD_ADDITIONALARGS) /p:BaseNuGetRuntimeIdentifier=ubuntu.14.04
-	DOTNET_PLATFORM = ubuntu-x64
+	MSBUILD_ADDITIONALARGS := $(MSBUILD_ADDITIONALARGS) /p:BaseNuGetRuntimeIdentifier=$(shell . /etc/os-release && echo $$ID.$$VERSION_ID)
 else ifeq ($(OS_NAME),Darwin)
 	MSBUILD_ADDITIONALARGS := $(MSBUILD_ADDITIONALARGS) /p:BaseNuGetRuntimeIdentifier=osx.10.10
-	DOTNET_PLATFORM = osx-x64
 endif
 
 ifneq ($(BUILD_LOG_PATH),)
@@ -89,6 +87,6 @@ $(TOOLSET_PATH): $(BINARIES_PATH)/dotnet-cli
 $(BINARIES_PATH)/dotnet-cli:
 	@mkdir -p $(BINARIES_PATH) ; \
 	pushd $(BINARIES_PATH) ; \
-	curl -O https://dotnetcli.blob.core.windows.net/dotnet/preview/Binaries/$(DOTNET_VERSION)/dotnet-dev-$(DOTNET_PLATFORM).$(DOTNET_VERSION).tar.gz && \
-	mkdir -p $(BINARIES_PATH)/dotnet-cli && \
-	tar -xzf dotnet-dev-$(DOTNET_PLATFORM).$(DOTNET_VERSION).tar.gz -C dotnet-cli
+	curl -O https://raw.githubusercontent.com/dotnet/cli/rel/1.0.0/scripts/obtain/dotnet-install.sh ; \
+	chmod +x dotnet-install.sh ; \
+	./dotnet-install.sh --version "$(DOTNET_VERSION)" --install-dir "$(BINARIES_PATH)/dotnet-cli"