gh-36454: Remove `tomli` from some build dependencies
    
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No longer needed because our current version of `flit_core` vendors it.

Also eliminates one use of `--no-build-isolation`.

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URL: #36454
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee

Author: Release Manager

build/pkgs/backcall/dependencies

@@ -1,4 +1,4 @@
- | $(PYTHON_TOOLCHAIN) flit_core tomli $(PYTHON)
+ | $(PYTHON_TOOLCHAIN) flit_core $(PYTHON)
 
 ----------
 All lines of this file are ignored except the first.

build/pkgs/entrypoints/dependencies

@@ -1,4 +1,4 @@
- | $(PYTHON_TOOLCHAIN) flit_core tomli $(PYTHON)
+ | $(PYTHON_TOOLCHAIN) flit_core $(PYTHON)
 
 ----------
 All lines of this file are ignored except the first.

build/pkgs/pyparsing/dependencies

@@ -1,4 +1,4 @@
- | pip wheel flit_core tomli $(PYTHON)
+ | pip wheel flit_core $(PYTHON)
 
 ----------
 All lines of this file are ignored except the first.

build/pkgs/tomli/spkg-install.in

@@ -1,7 +1,2 @@
 cd src
-# tomli's build system, flit_core, has a runtime dependency on tomli.
-# Hence we make the uninstalled tomli sources available during the installation
-# and disable build isolation -- which would require a tomli wheel to
-# be available for setting up the build environment.
-export PYTHONPATH="$(pwd)"
-sdh_pip_install --no-build-isolation .
+sdh_pip_install .