In many Linux distros such as NixOS, the directory in which packages are
installed is assumed to be read-only. To work around this, it is
expected that the filepaths which packaged CLI tools take in are able to
be freely configured through either 1) command-line flags, 2)
environment variables, or 3) a configuration file.
In this commit, environment variables 'BPF_SDK_PATH' and 'BPF_OUT_PATH',
which map respectively to command-line flags '--bpf-sdk-path' and
'--bpf-out-dir', are now handled in cargo-build-bpf.
Additionally, given that arbitrary filepaths may now be set in which the
BPF SDK is located, the requirement in which
'$BPF_SDK_PATH/dependencies/bpf-tools' must strictly be a symbolic link
to the directory '$HOME/.cache/solana/${bpf-tools.version}/bpf-tools has
been relaxed.
Ideally, the directory in which bpf-tools is expected to be downloaded
to and stored should be configurable. Though, this commit serves as a
temporary fix which enables NixOS users to now start being able to build
applications with the Solana SDK.
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