Use of new @bazel_tools//src/conditions targets breaks build on old Bazel versions

[jmillikin]

Example error:

ERROR: While resolving configuation keys for @m4_v1.4.18//gnulib:gnulib: no such target '@bazel_tools//src/conditions:openbsd': target 'openbsd' not declared in package 'src/conditions' (did you mean 'freebsd'?) defined by /github/home/.cache/bazel/_bazel_root/96335d2574d677ca0e31aa2073ba4534/external/bazel_tools/src/conditions/BUILD

Work to be done:

• Copy Github Actions configuration from rules_flex[0] to verify building with older Bazel versions.
• Change the select() matches to use targets not bundled into Bazel (@platforms// seems like the correct replacement).

[0] https://github.com/jmillikin/rules_flex/blob/e32e634a3f91837c66059f58aa0829f9b8caf1f4/.github/workflows/tests.yaml