pkg/golang: use GOROOT to find the correct compiler.
pkg/golang: use GOROOT to find the correct compiler.

Previously, if one set the GOROOT environment variable, we would not
actually use the the compiler found in $GOROOT/bin/go, but the compiler
found in $PATH.

build.Context.GOROOT is by default the compiled code's GOROOT, unless
the GOROOT environment variable is set.

GanShun/u-root,hugelgupf/u-root,u-root/u-root,rminnich/u-root,rminnich/u-root,rminnich/u-root,GanShun/u-root,u-root/u-root,GanShun/u-root,hugelgupf/u-root,hugelgupf/u-root,u-root/u-root,hugelgupf/u-root,rminnich/u-root