Linux build: need to unlink R when installing processx

Because after all package installations it is already too
late, by then callr has embedded client.so.

[ci skip]

tools/build/linux/Dockerfile

@@ -25,7 +25,7 @@ RUN apk add linux-headers bash gcc musl-dev g++ pkgconf patchelf \
 # Build binary package ----------------------------------------------------
 
 COPY pak_*.tar.gz /root/
-RUN R CMD INSTALL pak_*.tar.gz
+RUN PROCESSX_UNLINK_R=true R CMD INSTALL pak_*.tar.gz
 
 # Minimize library --------------------------------------------------------