Use dotnet 8 feed. #1797

Job	Run time
Build on windows-latest for repo global.json	20m 33s
Build on windows-latest for repo global.json	19m 3s
Build on windows-latest for 6.0 stable	19m 39s
Build on windows-latest for 6.0 stable	19m 39s
Build on windows-latest for 7.0 preview	26m 15s
Build on windows-latest for 7.0 preview	26m 15s
Build on macos-latest for repo global.json	27m 4s
Build on macos-latest for repo global.json	27m 4s
Build on macos-latest for 6.0 stable	29m 9s
Build on macos-latest for 6.0 stable	29m 9s
Build on macos-latest for 7.0 preview	27m 30s
Build on macos-latest for 7.0 preview	27m 30s
Build on ubuntu-latest for repo global.json	18m 4s
Build on ubuntu-latest for repo global.json	18m 4s
Build on ubuntu-latest for 6.0 stable	13m 4s
Build on ubuntu-latest for 6.0 stable	13m 4s
Build on ubuntu-latest for 7.0 preview	16m 8s
Build on ubuntu-latest for 7.0 preview	16m 8s
	6h 33m 22s

Provide feedback