Update Quick Start Build in User's Guide

[danrosen25]

Discussed in https://github.com/orgs/esmf-org/discussions/255

Update Quick Start section of User's Guide with information for using common build settings from user's environment.

export ESMF_C="$CC"
export ESMF_CXX="$CXX"
export ESMF_F90="$FC"
export ESMF_CLINKOPTS="$LDFLAGS"
export ESMF_CXXLINKOPTS="$LDFLAGS"
export ESMF_F90LINKOPTS="$LDFLAGS"

Also add sections for

• spack build
• install-esmf-action action (GitHub Runners)

[anntsay]

Spack build instruction is already added. just need the install-esmf-action action (GitHub Runners) instruction.