Initial support building the runner on FreeBSD

[ChanTsune]

I tried to address #385.

The dotnet binary for FreeBSD is not available on https://dotnetcli.azureedge.net, so I am using the system's dotnet.

I have adjusted the build process to support FreeBSD, following the examples of other operating systems.
The following steps have been tested and confirmed to pass successfully.

./dev.sh layout Release freebsd-x64
./dev.sh package Release

I have confirmed that ./dev.sh test passes, except for tests that require pre-built binaries such as node.
However, the operation of the runner on FreeBSD is not fully guaranteed, so please note that this is a PR that enables building for FreeBSD.

[er2off]

Generally LGTM but I made some changes

• In dev.sh shebang should be changed to #!/usr/bin/env bash for better cross-platform support (or use bash ./dev.sh).
  (This change also could be made for all other sh/bash run scripts)
  Also script fails to run after "Skip installing dotnet, use system dotnet"
  so you need to add mkdir -p "${DOTNETSDK_INSTALLDIR}" after (also someone from Microsoft, please add dotnet to Azure).

• Dotnet version provided in global.json doesn't exist in FreeBSD packages, there are only (at this moment)
  9.0.105 (package lang/dotnet)
  and 8.0.106 (package lang/dotnet8)

  Of course dotnet 9 fails to restore packages as target is net8.0, dotnet8 package seems OK to build everything.

• Tests depends on currently non-existed packages like _layout/externals/node20/bin/node and latest release of runner (for self updater test), this was already said.
  But I noticed that src/Runner.Listener/Configuration/ServiceControlManager.cs wasn't fully ported.
  Test said Missing ServiceLocatorAttribute for interface 'GitHub.Runner.Listener.Configuration.ILinuxServiceControlManager'. Add the attribute to the interface or whitelist the interface in the test.
  and that's true, service manager for FreeBSD init wasn't written in this PR.

• Just a note: self updater requires bash which isn't part of FreeBSD base system. As long as run scripts depends on bash (so you need to install it to make it work), this isn't important.