nixos/activation: avoid build failure with large activation script

Two improvements:
* use `writeShellApplication` (which uses `passAsFile` instead of
  passing the activation script as an env var into the derivation.
  We disable shellcheck and the bash options that this builder
  usually adds to avoid issues with out-of-tree activation scripts.
* use `sed` instead of `substituteInPlace`, since the substitute
  functions load the file content into a shell variable

This avoids issues when the activation is too long to fit in a shell variable.
Before this commit, a very large activation script, would cause build
failures because of different limits on the file content size, e.g.
```
➜ nix build -f . nixosTests.restartByActivationScript.nodes.machine.system.build.toplevel -vL
this derivation will be built:
  /nix/store/xw6anpfjyamnycjg58cmj01sz3ididyl-nixos-system-machine-test.drv
building '/nix/store/xw6anpfjyamnycjg58cmj01sz3ididyl-nixos-system-machine-test.drv'...
nixos-system-machine-test> error: executing '/nix/store/rlq03x4cwf8zn73hxaxnx0zn5q9kifls-bash-5.3p3/bin/bash': Argument list too long
error: builder for '/nix/store/xw6anpfjyamnycjg58cmj01sz3ididyl-nixos-system-machine-test.drv' failed with exit code 1;
       last 1 log lines:
       > error: executing '/nix/store/rlq03x4cwf8zn73hxaxnx0zn5q9kifls-bash-5.3p3/bin/bash': Argument list too long
       For full logs, run:
        nix log /nix/store/xw6anpfjyamnycjg58cmj01sz3ididyl-nixos-system-machine-test.drv
```

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