Try to build ucrt with rtools40 for backward compatibility

Author: jeroen

.github/workflows/R-CMD-check.yaml

@@ -31,7 +31,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      - if: runner.os == 'windows' && matrix.config.r == '4.1'
+      - if: runner.os == 'windows'
         uses: r-windows/install-rtools@master
 
       - name: Use precompiled libgifski

src/Makevars.win

@@ -16,8 +16,12 @@ export CARGO_TARGET_X86_64_PC_WINDOWS_GNU_LINKER=$(RTOOLS40_HOME)\\mingw64\\bin\
 export CARGO_TARGET_I686_PC_WINDOWS_GNU_LINKER=$(RTOOLS40_HOME)\\\mingw32\\bin\\gcc
 else
 export CARGO_TARGET_X86_64_PC_WINDOWS_GNU_LINKER=gcc
+ifdef RTOOLS40_HOME
+GCCPATH=$(subst C:\,/c/,$(RTOOLS40_HOME))/ucrt64/bin
+else
 GCCPATH=$(subst C:\,/c/,$(RTOOLS43_HOME))/x86_64-w64-mingw32.static.posix/bin
 endif
+endif
 
 all: clean rustup