Update agent methods to use the selected R interpreter consistently #3617

Job	Run time
home-view-unit-tests / lint	10s
build / build	1m 40s
lint / lint	1m 31s
agent / native (macos-latest)	49s
agent / native (windows-latest)	1m 52s
vscode / native (macos-latest)	1m 14s
agent / native (ubuntu-latest)	43s
vscode / native (windows-latest)	3m 3s
vscode / native (ubuntu-latest)	1m 16s
bats / native (ubuntu-latest)	4m 21s
archive / archive	35s
package / vscode	4m 23s
vscode-ui / ui-tests (ubuntu-latest)	5m 17s
upload / upload	1m 41s
	28m 35s

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