This commit removes the DYLD_LIBRARY_PATH variable from the default
modules.yaml for darwin. The rationale behind deleting this
environment variable is that paths in this environment variable take
precedence over the default locations of libraries (usually the
install path of the library), which can lead to linking errors in some
circumstances. For example, executables intended to link with Apple's
system BLAS and LAPACK will instead link to a spack-installed
implementation (e.g., OpenBLAS), causing runtime errors.
These errors are resolved by instead relying on paths set in
DYLD_FALLBACK_LIBRARY_PATH, which is lower in precedence than default
locations of libraries.
