spack install: simplify behavior when inside environments (spack#35206)

Example one:

```
spack install --add x y z
```

is equivalent to

```
spack add x y z
spack concretize
spack install --only-concrete
```

where `--only-concrete` installs without modifying spack.yaml/spack.lock

Example two:

```
spack install
```

concretizes current spack.yaml if outdated and installs all specs.

Example three:

```
spack install x y z
```

concretizes current spack.yaml if outdated and installs *only* concrete
specs in the environment that match abstract specs `x`, `y`, or `z`.