Update agent methods to use the selected R interpreter consistently #3626

Job	Run time
build / build	1m 41s
home-view-unit-tests / lint	13s
lint / lint	1m 30s
agent / native (macos-latest)	45s
agent / native (windows-latest)	1m 1s
vscode / native (macos-latest)	1m 5s
agent / native (ubuntu-latest)	48s
vscode / native (windows-latest)	4m 23s
vscode / native (ubuntu-latest)	1m 17s
bats / native (ubuntu-latest)	3m 56s
package / vscode	4m 9s
archive / archive	1m 25s
upload / upload	56s
vscode-ui / ui-tests (ubuntu-latest)	6m 24s
	29m 33s

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