Update go bootstrap to 1.15

go/build.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -eu
 
-export MAKEFLAGS="-j$(nproc)"
+export MAKEFLAGS="-j${NPROC:-$(nproc)}"
 
 [ ! -d build/install-bootstrap ] && ./build_bootstrap.sh
 [ ! -d build/install-go ] && ./build_go.sh

go/build_go.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -eu
 
-version_go=1.13
+version_go=1.15
 
 dir_download="$PWD/download"
 mkdir -p build; cd build

go/download.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -eu
 
-version_go=1.13  # https://golang.org/dl/
+version_go=1.15  # https://golang.org/dl/
 version_bootstrap=20171003  # https://golang.org/doc/install/source#go14
 
 mkdir -p download

go/download.sha256

@@ -1,2 +1,2 @@
-3fc0b8b6101d42efd7da1da3029c0a13f22079c0c37ef9730209d8ec665bf122  go1.13.src.tar.gz
+69438f7ed4f532154ffaf878f3dfd83747e7a00b70b3556eddabf7aaee28ac3a  go1.15.src.tar.gz
 f4ff5b5eb3a3cae1c993723f3eab519c5bae18866b5e5f96fe1102f0cb5c3e52  go1.4-bootstrap-20171003.tar.gz