Add support for specifying DEVELOPER_DIR as environment variable and env attribute value.

[cgrindel]

Related to #68.

• Added env attribute to spm_repositories.
• Added code to derive an env dict from the env attribute value and/or the DEVELOPER_DIR environment variable.
• Added repository_utils.exec_spm_command() to support running SPM commands. This function handles adding xcrun to the command line, if it is being executed on macos.
• Updated all SPM command executions to use the new utility, passing along the derived env.
• Updated the Github actions workflow with two additional tests:
  • Change default to incompatible Xcode and then override it with DEVELOPER_DIR environment variable.
  • Change default to incompatible Xcode and then override it with env attribute specifying the DEVELOPER_DIR to use.