It looks like ghc in Debian unstable is defaulting to new style cabal builds which bypasses Bootstrap.bootstrapWith Bootstrap.OSOnly:

git branch origin/spw3conf gpg signature verified; merging
Already up to date.
Build profile: -w ghc-8.8.3 -O1
In order, the following will be built (use -v for more details):
 - IfElse-0.85 (lib:IfElse) (requires build)
 - first-class-families- (lib) (requires build)
 - hsc2hs-0.68.7 (exe:hsc2hs) (requires download & build)
 - old-locale- (lib) (requires build)
 - syb-0.7.1 (lib) (requires build)
 - th-abstraction- (lib) (requires build)
 - network-bsd- (lib) (requires build)
 - type-errors- (lib) (requires build)
 - hslogger- (lib) (requires build)
 - propellor-5.10.1 (lib) (first run)
 - propellor-5.10.1 (exe:propellor-config) (first run)
Downloading  hsc2hs-0.68.7
Configuring IfElse-0.85...
Preprocessing library for IfElse-0.85..
Building library for IfElse-0.85..
[1 of 1] Compiling Control.Monad.IfElse ( Control/Monad/IfElse.hs, dist/build/Control/Monad/IfElse.o )
Downloaded   hsc2hs-0.68.7

I think the simplest fix is to replace cabal build with cabal v1-build in Propellor.Bootstrap.buildCommand?