cabal install run immediately after cabal build unnecessarily rebuilds the project

**Describe the bug**

I have just run cabal build. No source files have changed. I run cabal install (with some options), and it rebuilds the entire project from scratch.

**To Reproduce**

cabal unpack propellor-5.9.1
cd propellor-5.9.1
cabal build
cabal install --install-method=symlink --installdir=. exe:propellor --overwrite-policy=always

I notice that, if I `cabal build exe:propellor`, the `cabal install` does not re-build.
The default `cabal build` of both executables and the library differing from what's it's being asked to install seem to be what causes the re-build. I think it should be possible to build everything and then only install part of what was build, without a rebuild.

**System information**

cabal-install version 3.0.0.0
compiled using version 3.0.1.0 of the Cabal library

Reproduce the behavior with the listed cabal unpack, build, and install commands, comparing default cabal build with cabal build exe:propellor. Investigate how cabal install selects previously built executables and libraries. Done means installing exe:propellor after a full build does not rebuild unchanged project components.

Written by the indexing model from the issue text.