build: upgrade builder image to go 1.15.11

[knz]

We would like to pick the following fixes to the Go compiler and runtime system:

golang/go#45076

golang/go#45187

golang/go#42884

And also do this in the 21.1 release branch.

[blathers-crl]

Hi @knz, please add branch-* labels to identify which branch(es) this release-blocker affects.

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[knz]

Here are the places that need to be updated, if my reading is right:

WORKSPACE:go_register_toolchains(go_version = "1.15.10")
build/bootstrap/bootstrap-debian.sh:curl -fsSL https://dl.google.com/go/go1.15.10.linux-amd64.tar.gz > /tmp/go.tgz
build/builder/Dockerfile: && curl -fsSL https://storage.googleapis.com/golang/go1.15.10.src.tar.gz -o golang.tar.gz \
build/packer/teamcity-agent.sh:curl -fsSL https://dl.google.com/go/go1.15.10.linux-amd64.tar.gz > /tmp/go.tgz
pkg/cmd/roachtest/go_helpers.go:                ctx, c, node, "download go", `curl -fsSL https://dl.google.com/go/go1.15.10.linux-amd64.tar.gz > /tmp/go.tgz`,

And also build/go-version-check.sh as per #63837