ci: build with Go 1.13 on Windows

The default Go version is 1.12. Because Go 1.13 introduced language
changes (improved numeric constants), TinyGo compiled with Go 1.12
cannot handle Go 1.13 code such as the Go 1.13 standard library.

Use Go 1.13 to build TinyGo on Azure Pipelines to fix this.

Makefile

@@ -146,6 +146,7 @@ tinygo-test:
 
 .PHONY: smoketest
 smoketest:
+	$(TINYGO) version
 	# test all examples
 	$(TINYGO) build -size short -o test.hex -target=pca10040            examples/blinky1
 	@$(MD5SUM) test.hex

azure-pipelines.yml

@@ -50,14 +50,16 @@ jobs:
       inputs:
         targetType: inline
         script: |
-          export PATH="$PATH:./llvm-build/bin:/c/Program Files/qemu"
+          export PATH="/c/Go1.13/bin:$PATH:./llvm-build/bin:/c/Program Files/qemu"
+          unset GOROOT
           make test
     - task: Bash@3
       displayName: Build TinyGo release tarball
       inputs:
         targetType: inline
         script: |
-          export PATH="$PATH:./llvm-build/bin:/c/Program Files/qemu"
+          export PATH="/c/Go1.13/bin:$PATH:./llvm-build/bin:/c/Program Files/qemu"
+          unset GOROOT
           make release -j4
     - publish: $(System.DefaultWorkingDirectory)/build/release.tar.gz
       displayName: Publish tarball as artifact
@@ -67,5 +69,6 @@ jobs:
       inputs:
         targetType: inline
         script: |
-          export PATH="$PATH:./llvm-build/bin:/c/Program Files/qemu"
+          export PATH="/c/Go1.13/bin:$PATH:./llvm-build/bin:/c/Program Files/qemu"
+          unset GOROOT
           make smoketest TINYGO=build/tinygo AVR=0 RISCV=0