Use the isolated test env for shell commands

Shell.RunShellCommand was passing os.Environ() to its child, while its
sibling runCommandWithOutputAndEnv has used the minimal
NewTestEnvironment since late 2023 when env isolation was introduced;
the sh path was just missed.

This matters when integration tests run from inside a `git rebase -x`
exec in a linked worktree: git sets GIT_DIR=<main>/.git/worktrees/<name>
for the exec, and it leaks all the way down through bash, just, go test,
and the test process, into every git invocation RunShellCommand spawns.
cmd.Dir becomes irrelevant — git resolves GIT_DIR over cwd-based
discovery, with the work-tree taken from the gitdir file (i.e. the
worktree root). So `git checkout -b conflict` in a test fixture creates
the branch on the real worktree and switches its HEAD, hijacking the
in-progress rebase and trashing the working tree. (In the main worktree
git doesn't set GIT_DIR for rebase exec, which is why the bug was only
visible from linked worktrees.)

Using self.env also incidentally restores GIT_CONFIG_GLOBAL for shell
commands, so commits made via RunShellCommand are now authored by the
test config's CI identity rather than whatever the host's ~/.gitconfig
resolves to.

pkg/integration/components/shell.go

@@ -77,7 +77,7 @@ func (self *Shell) RunShellCommand(cmdStr string) *Shell {
 	}
 
 	cmd := exec.Command(shell, shellArg, cmdStr)
-	cmd.Env = os.Environ()
+	cmd.Env = self.env
 	cmd.Dir = self.dir
 
 	output, err := cmd.CombinedOutput()